An automatic spectrophotometric kinetic method is described for the ultramicrodetermination of iridium, based on its accelerating effect on the periodate photodecomposition. The time required for the reaction to consume a fixed amount of periodate is measured automatically and related directly to the iridium concentration. Ultramicroamounts of iridium in the range 80–800 ng (1.0×10−7-1.0 x 10−6 M) were determined with an average error of about 2%. The relative standard deviation for a 100-ppb iridium sample was 1.8%.