We demonstrate that the Cassie-Baxter model of contact angle for a composite surface consisting of "bare" and "covered" components (typically adventitious or purposeful carbonaceous deposits) can be used for quantitative determination of the kinetics of "self-cleaning" photocatalyst surfaces. In particular, literature data is analyzed to demonstrate examples of zero and first order reaction, as well as quantitative determination of rate constant values for two key processes: the initial activation of new photocatalysts, and the removal of purposeful fatty acid deposits such as that required in the ISO standard 27448: 2009 for activity determination of photocatalytic self-cleaning surfaces.