Many factors influence the persistence of traces of pharmaceuticals in aqueous environments. Most important are the intensity of light and the presence of inorganic ions as well as organic matter. We studied the impact of some environmental factors (humic acids, NO₃^- and NO₂^- ions, solar light intensity, and ambient pH) on the stability and kinetics of photo-reactions of paracetamol, naproxen, and diclofenac. It was stated that paracetamol was photoresistant, while naproxen and diclofenac were photoliable. An addition of NO₃^- and NO₂^- ions or humic acid strongly inhibited photodecomposition of paracetamol. Their presence in the solution of naproxen slowed down its decomposition, while in the case of diclofenac their influence on the kinetics of the photoreaction was neglected. The effect of the presence of the natural matrix on the photoreaction of the studied pharmaceuticals was checked. The influence of certified reference material – a water sample from the Grand River (Canada) and a sample of treated municipal waste – was examined. We observed that the matrix created by treated municipal waste acted as a photo-sensitizer. Its presence in solution accelerated the photodecomposition of all studied compounds.