Richard Hanks
Analyst · Goldman Sachs
Thank you, Jerry. We are very pleased to see the economic recovery playing out, as evidenced by our strong first quarter. I will review many of the key financial metrics but won't cover net income, earnings per share, debt ratios or standalone adjusted EBITDA as I would normally do so as a result of the pending noncash adjustments relating to the accounting for warrants, which, as Jerre mentioned earlier, has no impact on the operational performance of the company.
For the first quarter of 2021, adjusted revenues were $432 million, an increase of $189 million or 75% at constant currency compared to last year's same period. Excluding Techstreet, which we divested in early November 2020, adjusted revenue increased 81% at constant currency. The drivers of the growth were last year's acquisitions of DRG and CPA Global and an increase in organic revenue, partially offset by the Techstreet disposal. The foreign exchange impact on first quarter revenue was a positive 3.3% due to dollar weakness compared to last year's first quarter. And as Jerre mentioned, the strong start to the year has allowed us to bring up the low end of both, our 2021 revenue outlook by $10 million and our adjusted EBITDA outlook by $5 million.
Adjusted organic revenue growth was 7% at constant currency. Subscription revenue was $235 million, an increase of 19% at constant currency, driven by acquisitions and organic growth, partially offset by divested products. Organic subscription revenue growth was 6%, or $12 million at constant currency due to a favorable subscription entry rate, pricing and timing benefits from tighter operating procedures, enabling us to reduce overdue renewals by 85% compared to last year's first quarter.
The subscription revenue renewal rate at the end of the first quarter was 93%, in line with the 93% for the prior year period, but importantly, we have renewed a larger percentage of the book at the end of this year's first quarter as compared to prior year. Transactional revenue was $84 million, an increase of 68% year-on-year at constant currency, primarily driven by acquisitions and organic growth. Organic transactional revenues increased by $6 million or 10% at constant currency due to strength in our professional services businesses, including strong performances from DRG, and an increase in trademark search volumes in CompuMark. We continue to see a nice recovery in this segment of our business following the impact from COVID last year.
Reoccurring revenue, which is derived from the CPA Global patent renewals business, was $112 million in the first quarter, with no figure for the comparative period. Subscription plus reoccurring revenue accounted for 80% of adjusted revenues in the first quarter, demonstrating our highly predictable revenue model. ACV growth at constant currency was 11% for the first quarter as compared to the same prior year period, which includes acquisitions. Excluding divestitures, ACV growth was up 16%, while on an ongoing basis, ACV increased by 6% period-to-period, consistent with the 6% growth in organic subscription revenue growth all on a constant currency basis.
Turning to the business segments. Organic revenue growth within the Science Group increased by 10%, driven by new business, growth at DRG, which annualized into organic growth from March 2021 and tighter operating procedures, resulting in lower overdue renewals, which added to organic subscription revenue growth. For the IP Group, organic revenue increased by 2% on a constant currency basis, primarily due to an increase in subscription revenue, driven by content upgrades and better price realization as well as growth in transactional revenue due to improved transactional volumes.
Geographically, organic revenue growth was 7% at constant currency across each individual region, the Americas, EMEA and Asia Pacific. This reflects nicely the balanced recovery of our businesses following the challenges in 2020.
We delivered strong adjusted EBITDA growth in the first quarter, increasing by $87 million to $165 million, more than doubling as compared to the prior year period. This was driven by contributions from acquisitions and organic top line growth, strong margin flow through and the benefit of the cost savings initiatives. Adjusted EBITDA margin improved by 600 basis points to 38% from the same period prior year.
Per our 2021 outlook, we expect to see a sequential improvement in our margins throughout the year as we progress towards a 44% to 45% full year margin for 2021. Cash taxes in the first quarter were $3 million compared to $5 million in the prior year period. Capital expenditures in the first quarter were $33 million, an increase of $14 million over last year's first quarter, primarily due to the addition of DRG and CPA Global.
For the first quarter, adjusted free cash flow was $163 million, an increase of $85 million, more than doubling as compared to the prior year period, driven by the strong operating results and an improvement in working capital. We ended the March 31 Q1 2021 period with $399 million in cash and cash equivalents, an increase of $141 million from the year-end 2020. This was due to contributions from earnings and ever tighter working capital management. Our total debt is $3.5 billion, and decreased by $7 million from year-end 2020.
With that, I'll now turn the call back to Jerre.