Photolysis is a known dissipation pathway of pesticides on leaves just after their spraying. This pathway may be affected by the residues of other pesticides. To illustrate this idea, this study investigated the mutual effect of two pesticides (chlorothalonil and cycloxydim) under simulated solar light. Cycloxydim was added at the agricultural rate (200 g ha–1) and chlorothalonil at 1.3–10% of the rate (20–150 g ha–1). These compounds were studied either pure or in their commercial formulation. Both analytical and kinetic data show that chlorothalonil significantly accelerates the decay of cycloxydim on wax films, promoting its oxidation, even at the lowest tested dose. Conversely, cycloxydim does not affect the fate of chlorothalonil. Moreover, the detection of oxidized forms of wax alkanes in the extracts demonstrates that chlorothalonil may have also a degrading effect on the leaves’ constituents under the action of solar light.