Flow injection determination of residual chlorine in solution has been carried out spectrophotometrically by methyl orange decolorisation and by formation of a yellow complex with o-tolidine. The carrier streams are 0.180 mM methyl orange in pH 2 buffer solution or 0.01 M hydrochloric acid, and 3.0 mMo-tolidinium dichloride (in 2 M hydrochloric acid), respectively. A sampling rate of 288 samples per hour has been obtained with the latter reagent and the detection limit is 0.08 p.p.m. of chlorine. Influence of flow parameters such as flow-rate, tube length and diameter and the effect of interferents on the determination have been investigated. A study of the hyprochlorite-ammonium-o-tolidine reaction system has been performed and a method for the simultaneous determination of NH₄⁺ and OCl^– is described.