A mesoporous titania-coated Love wave sensor is used to monitor the real-time degradation of stearic acid through a photocatalytic reaction. The pollutant is deposited directly on the porous film. It leads, at first, to a decrease of the resonant frequency. Then, the device is placed under UV light and its frequency response is real-time monitored. After 12 hours under exposure, the frequency successfully reached its initial value. Acoustic sensors present many advantages compared to conventional measurement systems for photocatalytic activity: real-time response, small size, and low cost. The Love wave devices could allow to monitoring the degradation kinetics of other pollutants. They could also be a useful tool to study the influence of the UV wavelength on the degradation rate.