The mechanism of direct UV photolysis of p-arsanilic acid (p-ASA), a widely used veterinary drug, was revised by means of laser flash photolysis coupled with high resolution liquid chromatography – mass spectrometry (LC-MS). None of p-ASA triplet state or singlet oxygen was found to directly participate in the photodegradation of p-ASA as it was assumed in previous works. Here we demonstrate that the main primary photoprocess is a monophotonic ionization (ϕion 266nm = 0.032) leading to the formation of hydrated electron and corresponding anilinyl cation radical. These primary species react with dissolved oxygen yielding secondary reactive oxygen species. The final organic photoproducts, such as aminophenol and different dimeric products, originate from various reactions between these secondary species. The generation of inorganic arsenic, both As(V) and As(III), was also observed in agreement with previous works. For the first time we report the quantum yield of p-ASA photodegradation, which decreases from 0.058 to 0.035 with the excitation wavelength from 222 to 308 nm.