LOS ANGELES >> One of the rewards of watching TV online is not having to sit through as many commercials. Now the networks are chipping away at that little luxury.

CBS shows twice as many ads per show on its website as it did last year. The CW network shows four times as many. Dozens of shows from major cable networks now carry as many ads online as they do on TV. More shows will follow soon.

The online audience is still small compared with television, but it’s growing. Networks hope that by showing more ads, they can make about as much money per viewer online as they do on the tube.

It’s a change from the early days of online video. When ABC started putting full episodes of its shows online in 2006, fans could zip through the hourlong dramas “Lost” and “Desperate Housewives” in about 45 minutes. One short ad played a few times per show.

Limiting commercials kept people from going to unauthorized websites to watch pirated copies of shows. It also helped networks reach new audiences in college dorms and teenage bedrooms.

Now, as online audiences grow, networks see an opportunity to make more money. A recent episode of “Hawaii Five-O” carried 6 1/2 minutes of ads online. That’s less than the 16 minutes on TV but double what an hourlong show carried on CBS.com a year ago.

Online video has improved in recent years with faster Internet connections and better technology. That has led some people to give up on regular TV — and hefty cable bills that come with it — and watch only online. The websites of ABC and NBC and some cable channels offer a range of recent episodes online, as does Hulu, a site owned by the parent companies of ABC, NBC and Fox.

Other networks offer live sports online. ESPN puts events on ESPN3.com for viewers who get Internet service through certain providers. And NBC put hundreds of hours of live competition online during the 2008 and 2010 Olympics. NBC agreed Tuesday to pay $4.4 billion for the rights to televise the Olympics through 2020.

Watching your shows on a computer, of course, means being forced to watch the ads. On the tube, digital video recorders allow you to fast-forward through them.

Americans on average spent about 160 hours a month in front of the tube in early 2010 and only seven watching video on a computer or phone, according to the latest data from Nielsen Co. But online video is growing fast. According to online ad firm FreeWheel Media Inc., people watched 9 billion online videos from clients such as Fox, CBS and Turner in the last three months of 2010, an increase of about 50 percent from the previous quarter.

ZenithOptimedia expects online video ad revenue in the U.S. to grow 22 percent this year to $3.3 billion, compared with just 5 percent growth for all TV ads to $59.4 billion.