One of the big questions of the pending Yahoo-Microsoft search pact -- Will Yahoo replace its search on mobile devices with Microsoft's search? -- remains unanswered.

The Web portal could be dragging its feet on making a decision because Microsoft MSFT hasn't exactly been a home-run hitter in mobile search, analysts say.

When the companies unveiled their alliance on July 29, Yahoo YHOO Chief Executive Carol Bartz said Yahoo had the option of using Microsoft's mobile search technology. She hinted that the company leaned that way when she added that if Yahoo did drop its own mobile search, it would "free us up to really invest in the other areas of the mobile experience."

'Will Continue To Innovate'

Yahoo plans to merge its PC-based search operations with Microsoft, assuming the deal passes antitrust review. Yahoo and Microsoft are the No. 2 and No. 3 most-used search engines, but even combined they lag far behind leader Google GOOG.

Asked for an update on the question of mobile search, the company said via e-mail: "Yahoo will continue to innovate and own the mobile user experience.

"We will carefully consider whether and how to implement Microsoft's mobile search services, and our approach may differ on a country-by-country basis."

Analysts say Yahoo may want to keep its options open because of the huge growth potential of the mobile ad market. U.S. mobile ad revenue is expected to jump to $3.3 billion in 2013 from $648 million last year, market tracker eMarketer says.

Under the pact, Microsoft's search engine will be used on Yahoo sites while Yahoo sells the text-based search ads. The companies will share the ad revenue. PC-based search ads are the fastest-growing segment of the multibillion-dollar online ad market.

But Yahoo may be able to make more money by keeping its own mobile search, says Greg Sterling, principal at Sterling Market Intelligence.

"Exactly how big the (mobile search) market will be relative to the PC is unclear, but it will certainly be substantial," Sterling said.

In June, Yahoo ranked No. 2 in mobile search with more than 5.5 million visitors, up from 4.2 million a year ago, says Nielsen Co., a research firm.

Google led with 22.4 million mobile search visitors. No. 4 Microsoft lagged behind with just over 2 million visitors, though that was up from 1.6 million a year earlier. IAC/InterActiveCorp.'s IACI Ask was No. 3.

In any case, mobile is a whole different market from PC search, says eMarketer analyst Noah Elkin.

"Yahoo is considerably further ahead in mobile than Microsoft," he said.

Slow To Join Mobile Search

In 1999, Microsoft's MSN Internet portal became the first to offer Web-based services such as news, sports and stock quotes for mobile devices. But its mobile search didn't arrive until 2005. Yahoo was a year ahead of it on that score.

Microsoft fell behind by failing to offer more and better-integrated services on mobile devices, Elkin says.

"Microsoft was not as quick to market with updates, and in tying it all together as quickly as others," he said.

In mobile, Microsoft has come up short, agrees Vanessa Alvarez, an analyst at research firm Frost & Sullivan.

"The platform just is not gaining traction," she said.

And unlike Yahoo and Google, Microsoft's online business is losing money.

Microsoft reported that its online business unit had an operating loss of $732 million for the quarter ended June 30, as revenue fell 14% to $837 million.

Microsoft has a huge cash hoard, however, and says it will spend $100 million to market its new Bing Web search service. So far, the ads are geared toward PC users.

Alvarez said Microsoft may feel it's fallen so far behind in mobile search that instead of playing catch-up, "they'll choose another area where they are able to compete better and focus their R&D."

Asked for comment, Microsoft said in an e-mail that Bing already lets users get information such as road directions, movie times, traffic maps and weather conditions on mobile devices. And more is coming.

"We are committed to delivering great Bing experiences on mobile devices," the company said. "You will see more innovation in this space from us soon."