Google (NSDQ: GOOG) AdWords has been one of the search engine’s most lucrative products on the wired web, but it may not be able to replicate its success on the mobile web since AdWords’ cannot deliver accurate data, according to Farhad Divecha, MD of London, UK-based search engine marketing agency AccuraCast. Moreover, for Adwords to ever give advertisers accurate data, Google will have to partner up with operators who control the information.

As Divecha explained, Google Adwords conversion tracking uses JavaScript which is placed by advertisers on their web site’s conversion, or customer acquisition page, to track which clicks have resulted in a conversion. The data is then reported back to the advertiser in the form of the number of conversions per keyword. While this works just fine on the wired internet, it doesn’t work as well on mobile phones, resulting in faulty data.

A Google account manager admitted to Divecha that “a significant percentage of mobile browser and carrier combinations do not support cookies. Google adds cookies to a user’s mobile device when he or she clicks on an ad to track conversions. Therefore, if users are using mobile browsers or carriers that do not accept or support cookies, they will not be included in your conversion tracking statistics.” The account manager also warned that cookies on mobile phones expire faster than on ones created on computers, and therefore “a significant number of conversions for your site may go unrecorded after a certain period of time.” Finally, she added, “Although Google cannot record every conversion due to the reasons mentioned above, your conversion rate, cost-per-conversion, cost-per-transaction and value/click are adjusted to reflect only those sites from which we can track conversions.”

As Divecha points out, if a “significant percent of mobile browsers and carrier combinations do not support cookies” the ROI calculations many mobile advertisers make for their campaigns won’t be accurate. “This is a big issue for advertisers. Advertisers are getting no visibility on which of the ad words they bought was the most effective; the tracking is way off,” Divecha told mocoNews. He noted that it was routine to serve up 20 different versions of ads for a keyword to figure out the maximum conversions. With mobiles, where space is limited, he notes a single keyword “could make or break your ad.” He added, “The operators are keeping all the toys to themselves. This could turn out to be big for the operators. All points lead to Google winning the race [in the mobile web]—but it’s the operators who know who the users are.”

Of course, this isn’t much of a problem now, with the mobile web in its infancy. With only a limited number of click-throughs on mobile ads anyway, not many are pouring over the analytics of their mobile campaigns. But as the mobile web begins to grow, and the price of AdWords are pushed up, an advertiser, says Divecha, is going to question “spending 100K on an AdWord campaign only to get £20 worth of conversions.”