The paper presents a spectrophotometric study of the binary system Rh(III)-2-thioxo-4-thiazolidinone. The formation conditions and the analytical application of the formed complex for spectrophotometric determination of Rh(III) were studied. The composition of the Rh(III) chelate with this particular ligand was determined by the continuous variation method, its instability constant being Kinst = 1,61·10-8 mol2L-2.