mechanism and the isomerism of the obtained products are discussed. According to the data of 1 H NMR spectroscopy the stabilized isomer is that in which the electron-withdrawing aryl substituent is located in position 3 and the electron-donating substituent in position 5 of the pyrazole ring.It was found previously [1] that acylhydrazones of β-dicarbonyl compounds are cyclized into pyrazoles as a result of condensation of the carbonyl group of the acylhydrazone fragment and an activated methylene group, when a methyl or phenyl substituent is present on the nitrogen atom next to the carbonyl group of the hydrazone fragment. Later [2] we showed that aroylhydrazones of 2-phenacyl-1H-benzimidazole in the absence of such substituents also undergo an analogous cyclocondensation with the formation of 2-(3-aryl-5-phenyl-1H-pyrazol-4-yl)-1H-benzimidazoles on heating to 200 o C. In this case it is possible to combine the preparation of the hydrazones and their cyclocondensation in one process, however the reaction proceeds only with benzoylhydrazine and with its derivatives containing electron-withdrawing substituents in the benzene ring. Up to the present time we have produced new experimental data which indicate the broader possibilities of this reaction and enable elucidation of its mechanism.We have found that the interaction of a series of 2-phenacyl-1H-benzimidazoles 1a-e with 4-nitrobenzoylhydrazine 2a, proceeding through hydrazones 3a-e and their subsequent cyclocondensation, leads to 2-[5-aryl-3-(4-nitrophenyl)-1H-pyrazol-4-yl]-1H-benzimidazoles 4a-e. Derivatives 4a,c,d with various aryl substituents in positions 3 and 5 of the pyrazole ring therefore exist in DMSO-d 6 solution in equilibrium with the tautomeric forms 4'a,c,d.The reaction occurs smoothly on heating in DMSO at 150 o C for 1 h with triethylamine hydrochloride as catalyst.4-Methoxybenzoylhydrazine 2b, the electrophilicity of the carbonyl group of which is reduced due to the electron-donating effect of the MeO group, reacts under more rigid conditions. The interaction of reactants 1a and 2b occurs at 200 o C, in the presence of a small amount of diglyme preventing crystallization of