We present a field experiment that uses geo-referenced smartphone advertisements to measure ethnic preferences at a highly disaggregated level. Different types of banners advertising a vote matching tool are randomly displayed to mobile Internet users in Bosnia and Herzegovina, while recording their spatial coordinates. Differences in the response (click) rate to different ethnic cues on these banners are used to measure temporal and spatial variation in ethnic preferences among the population of Bosnia and Herzegovina. Our study lays out the theoretical and practical underpinnings of this technology and discusses its potential for future applications, but also highlights limitations of this approach.