3- and 4-substituted 5-phenylazo derivatives of 2,4-thiazolidinedione were prepared as potential biologically active fungicides. The kinetics of coupling of 4-sulfobenzenediazonium salt with 2,4-thiazolidinedione and 3-methyl-2,4-thiazolidinedione have been studied and the results have been compared with kinetic data of coupling reaction of 3-methyl-5-isoxazolone.