The reaction of both aliphatic and aromatic thioamide derivatives with N-substituted maleimides under various conditions leads to the formation of complex polycyclic systems [1,2]. No information is available on the reaction of N-arylmaleimides with heterocyclic thiocarboxamide derivatives.We have found that the reaction of 1-aminothiocarbonyl-3,5-diarylpyrazolines 1a-1d [2] with N-substituted maleimides upon heating for 10 min in acetic acid at reflux leads to N-aryl-2-[2-(3,5-diaryl-4,5-dihydro-1H-pyrazol-1-yl)-4-oxo-4,5-dihydro-1,3-thiazol-5-yl]acetamides 2a-d in high yield. This reaction permits broad variation of the substituents R, R 1 , and R 2 in the pyrazolinothiazolone molecules 2, which allows us to synthesize large combinatorial libraries of such compounds. N N N H 2 S R R 1 N O O R 2 N N N S O NH O R 2 R R 1 AcOH 1a-d 2a-d He Hd Hf Ha Hb Hc 1 a R = R 1 = Ph, b R = 4-MeOC 6 H 4 , R 1 = 4-MeC 6 H 4 , c R = 4-FC 6 H 4 , R 1 = 3-ClC 6 H 4 , d R = 2-C 4 H 3 S, R 1 = Ph; 2 a R = R 1 = R 2 = Ph, b R = 4-MeOC 6 H 4 , R 1 = 4-MeC 6 H 4 , R 2 = 4-F, 3-ClC 6 H 3 , c R = R 2 = 4-FC 6 H 4 , R 1 = 3-ClC 6 H 4 , d R = 2-C 4 H 3 S, R 1 = Ph, R 2 = 3-MeOC 6 H 4The structure of products 2a-d was indicated by 1 H and 13 C NMR spectroscopy and mass spectrometry. The 1 H and 13 C NMR spectra were taken on a Varian Mercury VX-200 spectrometer at 200 and 50 MHz, respectively, in CDCl 3 with TMS as the internal standard. The IR spectra were taken on a Perkin Elmer One FTIR spectrometer for KBr pellets. The electron impact mass spectra were taken on a Varian 1200L mass spectrometer at 70 eV.