For the next five weeks, US firm Unicast is running 30-second full-screen, TV-quality video adverts on 15 major websites.
The ads, which appear at random as users move between pages, are displayed across the entire browser - but can be turned off by a click in a box.
The sites picked for the trial include MSN, ABC News, iVillage and Lycos.
For users already annoyed by the vast amounts of advertising on the web, Unicast argues that its adverts could lessen the bombardment of commercial messages.
Spokeswoman Allie Savarino told BBC News: "If you can offer formats that advertisers place a premium on, like television commercials, the amount of money they are willing to pay for that will increase.
"Therefore the number of ads a website will have to run in order to remain viable will decrease."
The "video commercial" format uses Microsoft's Windows Media Player, combined with pre-caching technology that downloads the advert into an internet browser.
Unicast says the format is up to eight times faster than broadband video, and promises an end to buffering and freezing.