ToF-SIMS was used to investigate the problem of yellow discoloration of white drug tablets upon light exposure. The data showed that selective degradation of the drug molecule had taken place where an acetic acid group was converted to an aldehyde group (mass difference = 30). This resulted in the formation of a new molecule (Mdeg) with a mass of 30 less than the pristine molecule (Mdrug) where M = molecular mass. Conjugation of this aldehyde group with an aromatic ring resulted in a shift in the light absorption from the ultra-violet region of the electromagnetic spectrum into, at least partially, the blue region of the visible spectrum, resulting in the yellow discoloration.