The interaction of Ru(IV) ions with a novel analytical reagent - 5-hydroxyimino-4-imino-1,3-thiazolidin-2-one, by spectrophotometric method was investigated. The complex is formed at pH 5.0 in acetate buffer medium after heating in the boiling water bath (~371 K) for 25 min. The complex has maximum absorption at 350 nm and is stable for 24 h. Beer's law is valid over the concentration range of 0.5-6.1 μg mL-1 for Ru(IV). The molar absorptivity at μ = 350 nm is 6.21 × 103 L mol-1 cm-1. The limit of detection of this method is 0.2 μg mL-1. The interfering effect of various cations and anions on the spectrophotometric determination of the Ru(IV) were investigated. The proposed method was successfully applied to the determination of Ru(IV) in alloys.