Propylene glycol, also called propane-1,2-diol, is a humectant and applied in topical solutions to increase the solubility of the active pharmaceutical ingredient (API) or the ability to penetrate the skin. In this application, propylene glycol is determined in a placebo and two topical solutions containing diclofenac as the API. The separation is achieved on a Hamilton RCX-30-250/4.0 column with subsequent pulsed amperometric detection (PAD).