TV giants like Turner and Viacom have been experimenting with cutting down the number of commercials they air in shows. They theorize that viewers will be happier, and they could even charge more for fewer ads, increasing revenue in the long run.

But so far, there is no clear evidence of a financial payoff.

Viacom attributed a 2% slide in domestic ad sales in the June quarter to a decision to reduce ad loads on networks such as MTV and BET. The company also warned that domestic ad sales will continue to decline in the September quarter.

Viacom’s hope is that over time its reduced clutter will win over viewers who otherwise skip through ads or change the channel or gravitate to ad-free platforms like Netflix.

Time Warner’s Turner cable unit said in late 2015 that it would reduce the number of commercials on its TruTV network the following year. The company, which was initially asking for a 20% to 30% premium for some of the scarcer inventory, said early last year that it planned to cut ads on TNT by 50% in three dramas.

Donna Speciale, president of Turner Ad Sales, declined to comment on the revenue impact of the overall ad load reduction efforts. But she acknowledged that for some shows the company isn’t charging a premium that is enough to offset the loss of ad slots.

“We had to eat some of this,” she said. “We were going to have to take a little bit of a hit.”

Cutting ad load has become a popular move in the TV world for networks coping with sweeping changes in consumer habits and ratings pressure.

Some are experimenting with new ad formats, including episode sponsorships and brand integrations with fewer standard ads.

Comcast Corp. ’s NBCUniversal last April said it would reduce ads on “Saturday Night Live” by 30%.

The network declined to comment on the pricing of limited ads, or the impact on ad revenue.

Fox, which planned to reduce commercial minutes by 20% during the Teen Choice Awards that aired on Sunday, said that as of Friday it booked 30% more revenue than last year’s haul from the event through the implementation of new ad products, including six-second spots that sold for a premium. The company declined to comment on the financial impact of ad cuts on other programs. (Fox parent 21st Century Fox and Wall Street Journal parent News Corp share common ownership.)

At MTV, viewership has eroded significantly over the past several years. But Viacom Chief Executive Bob Bakish said during the earnings call that the network has recently regained some ground, “marking the first June year-over-year ratings increase since 2011.”

“Our ad loads, in my opinion, were unhealthily high and therefore we have taken this opportunity to use this rating growth to partially fund a reduction in ad load,” Mr. Bakish said.

It isn’t clear whether Viacom has been asking for premiums on its more limited ad inventory.

Bernstein analyst Todd Juenger cautioned Viacom investors that the headwinds from reducing ad load aren’t over, urging them “to resist believing that after another quarter or two this will be ‘fixed and finished.’”

At the same time, he praised the efforts to avoid ad clutter.

Some advertisers are hesitant to pay a premium for commercial spots that are longer or more exclusive because of reduced ad load. Their main interest is in a show’s ratings.

“The reality is the reduced commercialization has not translated to incremental rating points,” said Gibbs Haljun, a managing director of media investment at WPP ad-buying network GroupM.

MTV reduced its monthly commercial time from 1,436 minutes in February 2016 to 1,284 minutes in February 2017, according to Nielsen data provided by an ad-buying agency.

Despite the reduced commercial load, MTV’s average audience among adults 18 to 49 fell 18.5% during the period. The figures include live viewing plus three days of recorded viewing.

Turner’s TruTV reduced its commercial time from 1,150 minutes in February 2016 to 1,044 in February 2017. Ratings were flat at the network. There are many factors that can affect ratings on various shows, and there is no explicit correlation between ratings and ad load.

Ms. Speciale said there were plenty of advertisers willing to pay up for the inventory and she is hopeful that new ad formats will help the network’s case.

Advertisers on TruTV at TNT enjoyed lifts in brand awareness of 30% and 39% at the respective networks following cuts in commercial time that allowed them to stand out more, according to the company’s research, which is largely based on data from Tapestry Research.

“The goal for me is to cumulate the results as much as possible so everyone realizes this is real, that the ad effectiveness is true value,” Ms. Speciale said.

A couple of ad buyers called reduced ad-load results inconclusive and said marketers could be persuaded over time to pay a premium.

“Many clients are happy to pay for twice the impact in a linear fashion if you can prove that it has twice the impact,” said Craig Atkinson, chief investment officer at Omnicom media agency PHD. “The problem is proving it. In the absence of proof, it’s difficult to have that conversation.”

Write to Alexandra Bruell at alexandra.bruell@wsj.com