The effect of structural evaluation of 4-thiazolidinone derivatives, i.e. 2,4-thiazolidinedione, 2- thiono-4-thiazolidinone, 2-imino-4-thiazolidinone, and 2-imino-4-oxo-5-thiazolidineacetic acid, is discussed. The highest protonation constant values were found for 2,4-thiazolidinedione and 2-thiono-4-thiazolidinone, due to deprotonation of the cyclic imine group. Basic properties increase in the following sequence: (=NH) < (-CH2COOH) < (=S) < (=O). The competitive ion flotation of Zn(II) and Cd(II) ions from diluted (c(Me) = 1.0 x 10(-5) M) aqueous solutions by the anionic surfactant, i.e. sodium dodecylbenzenesulfonate, in the presence of 4-thiazolidinone derivatives is studied. The most effective separation of Zn(II)/Cd(II) was found for a ligand with basic properties, i.e. 2-imino-4-thiazolidinone. The linear relationship between selectivity coefficients of Zn(II)/Cd(II) and first protonation constants was found for 2,4-thiazolidinedione, 2-thiono-4-thiazolidinone and 2-imino-4-oxo-5-thiazolidineacetic acid and was used for estimation of the K, value for 2-imino-4-thiazolidinone. (C) 2002 Elsevier Science Ltd. All rights reserved.