Some 176 million US viewers - 83.3 percent of the country's internet audience - tuned in to watch video online in May 2011.
Internet users watched an average of 15.9 hours of video during the month, up from 14.9 hours per viewer in April 2011.
A second study by comScore on the online video viewing habits of Europeans found that Germans watched the most hours of online video in April.
Internet users in Germany averaged 19.6 hours of online video viewing each per for the month, while internet users in Turkey, Spain, the UK, France and Italy averaged 18.7 hours, 18.4 hours, 17 hours, 12.8 hours and 12.8 hours respectively.
Google Sites (including YouTube.com) ranked as the top online video content property in the US for the month with 147.2 million unique viewers.
"Google Sites had the highest number of viewing sessions with more than 2.1 billion, and highest time spent per viewer at 311 minutes, crossing the 5-hour mark for the first time," said market researcher comScore, Inc. in its report on the US online video rankings for May.
VEVO came second in comScore's rankings, attracting 60.4 million unique viewers for the month, each visiting the site to watch an average of 105.1 minutes of video. Yahoo! Sites, Facebook.com and Viacom Digital followed attracting 55.5 million, 48.2 million and 46.5 million total unique viewers for the month respectively.
Ads made up a significant portion of viewers' online video experience, accounting for 12.6 percent of all videos viewed or 1.2 percent of all minutes spent viewing video online, said comScore.
The average online video lasted for 5.2 minutes while the average online ad lasted for just 24 seconds.