As mobile search traffic increases, mobile search ads are showing dramatically improved conversion rates, but the increases aren’t evenly split between device OS, reports Adobe. According to the company’s Q3 2012 digital advertising update, the iOS conversion rate is nearly double that of Android.

Using Android rates as the baseline, Adobe found U.S.-based iOS users demonstrated an 181 percent conversion rate on mobile search ads. European iOS users were even more likely to convert than their Android owning counterparts, with U.K. and German users showing a 193 percent conversion rate on iOS compared to Android. The categories of ad that showed the best conversion rates were finance, travel and retail.

Interestingly, although iOS had much higher click-through and conversion rates Adobe also found the overall cost-per-click rate was 18 percent lower on iOS. According to Adobe, the combination of these factors means placing a mobile search ad on iOS will generate twice the ROI as placing it on Android.

The news is interesting for marketers, given the fact that Adobe predicts one in five paid search clicks will originate from a smartphone or tablet during Q4 2012.