A kinetic method for the determination of trace amounts of mercury based on the marked catalytic effect exerted by Hg^II on the photochemical reaction between iodine and EDTA is proposed. The reaction, involving 6 × 10^–5M l₂ and 2 × 10^–4M EDTA, is carried out by irradiation with 2000 lux at pH 5 and 30 °C. The rate of the reaction is monitored amperometrically by measuring the time required for cancellation of the reduction current of iodine, measured with a Pt reference electrode maintained at –100 mV vs. SCE. Mercury can be determined in the range 0.4–6 ng ml^–1 by irradiation at pH 3 and 40 °C, and between 4 and 42 ng ml^–1 at pH 5 and 40 °C, using 2000 lux radiation in both instances. The proposed method has been applied with excellent results to the determination of mercury in zinc concentrates and in contact lens cleaning solutions.