Magazine publishers are seeing more advertisers warm up to their iPad editions.

In the first quarter, the number of ad units in magazines' iPad editions jumped 23.6% from the same period in 2012, according to a report from Kantar Media and the Publishers Information Bureau. The report, which looked at 58 magazines with monitored print and iPad editions, found that the number of ad pages in the titles' print editions was roughly flat in the first quarter, with 10,707. But the number of iPad ad units climbed to 5,961 in 2013 from 4,824 the year prior. Total print ad pages and iPad ad units combined increased 7.5% year-over-year, the report said.

Magazines reported paid and verified digital circulation of 7.9 million in the second half of last year -- just 2.4% of their total print and digital circulation, according to the Alliance for Audited Media. That number does not include every non-replica edition because some publishers don't report them to the AAM.

One impediment for iPad ads is cost. There are more production costs required for interactivity, and some publishers, including Condé Nast and Meredith, have charged advertisers for interactive elements.

But more print advertisers are opting in, said Chris Kevorkian, chief marketing officer at the MPA-Association of Magazine Media. "There's real growth in marketers saying "Let's move on, let's go into this platform.'"

Tablet ownership in the U.S. has nearly doubled in the past year, with 34% of American adults owning a tablet, up from 18% a year ago, according to the Pew Research Center. With 56% of these tablet owners living in households that earn at least $75,000 annually, publishers are pitching brands on reaching affluent audiences.

Bloomberg Businessweek offers advertisers several ways to appear on its iPad and iPhone app. The cost of a quarterly sponsorship -- which includes a full-page ad, a "brought to you by" logo and various banner ads -- is $85,000. IPad advertisers are given certain performance metrics on their ads, such as time spent and content downloads. Microsoft Dynamics and Credit Suisse USA are among the brands to sponsor sections. The magazine plans to offer monthly sponsorships starting in the third quarter of this year.

"There's a certain first-mover status for advertisers on the tablet," said Hugh Wiley, publisher, Bloomberg Businessweek. The magazine does not bundle print and iPad ads, meaning advertisers who want to appear in both must pay separately. The open rate for a full-page ad in the weekly's worldwide edition is $161,600.

Mr. Wiley said Bloomberg Businessweek does not discount its mobile or iPad ads, because there is limited inventory and high demand. "IPad is a big revenue driver for us," he said. The publication, which in May had more than 225,000 mobile- and tablet-app subscribers, saw first-quarter ad revenue on the app grow 147% from a year earlier.

But while the platform is gathering steam, buyers said publishers still need to provide more ad-performance data.