The photochemical decomposition of some aryldiazonium chlorides stabilised as zinc chloride double salts and incorporated in secondary cellulose acetate film has been studied by determining concentrations of residual salts after irradiation with a xenon‐arc lamp. Residual concentrations were measured by coupling the undecomposed diazonium salt to form coloured compounds, which were then estimated by visible‐light microspectrophotometry. Rates of fading for these salts indicate first‐order kinetics. Quantum yields are approximately 0–2, with the exception of those forp‐monoalkylamino derivatives, which are appreciab