A sensitive and selective colour reaction for the determination of nitrite is presented. Nitrite reacts with p-aminophenylmercaptoacetic acid in the presence of hydrochloric acid to form a diazonium cation, which is subsequently coupled with N-(1-naphthyl)ethylenediamine dihydrochloride in acidic medium to form a stable bluish violet azo dye. The method is suitable for the determination of nitrite from 0.02 to 0.80 p.p.m. in a 1.0-cm cuvette. The observed molar absorptivity and Sandell's sensitivity of the azo dye are 4.65 × 104 l mol–1 cm–1 and 0.001 µg cm–2, respectively. The method is free from most interferences. The method has been applied to polluted water samples and the results obtained compare favourably with those from standard methods.