This study is based on the oxidation of Pyrogallol Red (PGR) by cerium(IV) under the following conditions: pH 5, temperature 25 °C and at a wavelength of 507 nm. The decrease in absorbance of a 6 × 10–5 M PGR solution at a fixed time is proportional to the concentration of cerium(IV) over the range 0–15 p.p.m.; the limit of detection of cerium is 0.50 p.p.m.; the stoicheiometry of the system is 2:1 [cerium(IV): PGR]; and the reaction is first order with respect to cerium(IV). This paper describes studies on the selectivity of the method with respect to possible interference from 26 species, which have been classified according to their possible mechanism of interference. The system has a relative standard deviation in the range 3.1–0.3% for concentrations of cerium(IV) between 2 and 15 p.p.m. It does not present a constant-type error and does not require a blank correction at a fixed time of 5 min.