More than eight in 10 Internet users also do some offline activity while online, according to Burst Media.
Burst found that nearly three out of five Internet users watched television while online.
"TV and the Internet have long been multitasking buddies," said Debra Aho Williamson, senior analyst at eMarketer. "It makes sense to create ad messages that resonate across media."
The second most common offline activity for online multitaskers was job-related activities (33.0%), followed by reading a book (31.1%), a magazine or newspaper (29.7%) and talking on a mobile phone (23.6%).
"Keep in mind that many of these activities are not representative of true multitasking," Ms. Williamson said. "It's one thing to go online with the radio playing in the background, but it's all but impossible to read text on a Web page and in a newspaper at the same time. Consumers are switching focus, rather than multitasking."
"Still, anytime your attention is divided, your ability to recall and comprehend ad messages is impacted."
The Web is not the only medium to which consumers are giving divided attention. Music listeners and instant messagers were even more likely than computer users to be juggling media consumption, according to a Bridge Ratings study.
Learn how to reach college students—the biggest media omnivores. Read eMarketer's College Students Online: A Parallel Life on Social Networks report.