Detection of phenylacetone is of interest as it is used as a precursor for the synthesis of (meth)amphetamine. The ability to detect illegal drugs at e.g. border crossings is valuable for governments. Capacitive Micromachined Ultrasonic Transducers (CMUTs) have previously been used to detect small amounts of analyte. If the sensitivity of the CMUT is known, the resonance shift can be used to directly calculate the added mass and hereby the amount of an analyte. This typically requires a selective functionalization layer on top of the CMUT in order to differentiate between analytes. However, for some analytes such a layer can be difficult or impossible to obtain. In this work we use a colorimetric dye which is selective towards phenylacetone in conjunction with a CMUT to detect and quantify the analyte.