The kinetics of the uncatalysed and silver-catalysed oxidation of pyrogallol red (PGR) by peroxodisulphate in the presence of 1,10-phenanthroline (phen) as an activator were investigated. Rate equations for these reactions were derived from the kinetic dependences on hydrogen ion and reagent concentrations. The rate constants and the orders of reaction were also calculated. A kinetic model is proposed. The optimum conditions for silver(I) determination are as follows: pH 2.0 (in a sulphuric acid medium); S₂O₈^2–, 2 × 10^–2 mol dm^–3; phen, 8 × 10^–3 mol dm^–3; PGR, 3 × 10^–5 mol dm^–3; and temperature, 25 °C. Under these conditions and using the fixed time and initial rate methods, silver can be determined in the range 0.85–21.36 ng ml^–1. The regions of analyte measurement are as follows: analyte not detected, <0.26 ng ml^–1 of silver(I); region of detection, 0.26–0.88 ng ml^–1 of silver(I); and region of determination, >0.88 ng ml^–1 of silver(I). The relative standard deviation of the method at a fixed time of 300 s was 3.9–1.4% for silver concentrations from 0.85 to 15.10 ng ml^–1. The influence of 25 foreign species was investigated.