Keywords: dimethyl acetal of DMF, 2-(4-pyrazolyl)benzimidazoles, N-methylation.Previously [1] we assigned the structure 2a,b to the products of the reaction of 2-(1-aryl-5-phenylpyrazol-4-yl)-1H-benzimidzoles 1a,b with the dimethylacetal of DMF. This conclusion did not contradict the 1 H NMR spectral data, the elemental analysis, and information [2] on the existence of polycondensation of heterocyclic systems of this type. The result appeared to be unusual on the background of known [3][4][5] properties of the dimethylacetal of DMF. N N N N H Ar Ph N N N N Ar H OMe Ph N N N N Me Ar Ph Me 2 NCH(OMe) 2 -MeOH, -HCONMe 2 1a,b 3a,b 2a,b 1-3 a Ar = Ph, b Ar = 4-O 2 NC 6 H 4Over time, we had doubts about the correctness or our conclusions. We carried out additional chromatomass-spectroscopic analysis and detected that the molecular mass did not coincide with that calculated for the structure 2a,b. A second elemental analysis was carried out after thorough purification of the products. As a result it became evident that our original suggestion of the structure of the products was erroneous. In particular in the reaction cyclocondensation does not take place, but N-methylation at the benzimidazole nitrogen atom with formation of the compounds 3a,b, which accords with the known properties of the dimethylacetal of DMF [5].1 H NMR spectra of DMSO-d 6 solutions with TMS as the internal standard were recorded on a Varian VXR-300 (300 MHz) instrument. Chromatomass spectrometry was carried out on an Agilent 1100 series high resolution liquid chromatograph equipped with an Agilent LC MSD SI ( Parameters: Zorbax SB-C18 column, 1.8 µm, 4.6 mm, 15 mm, solvent acetonitrile-water (95:5), 0.1% trifluoroacetic acid, rate of elution 3ml/s; 1 µl sample injected; chemical ionization at atmospheric pressure with simultaneous scans of positive an negative ions with masse from 80-1000 m/z).