Formylation of 5-methyl-7-phenyl-4,7-dihydro-1,2,4-triazolo[1,5- a]pyrimidine 1a using Vilsmeier-Haack conditions yields 5-methyl-7-phenyl-4,7-dihydro-1,2,4-triazolo[1,5- a]pyrimidin-6-ylcarbaldehyde 3a. 5,7-Diaryl-4,7-dihydro-1,2,4-triazolo[1,5- a]pyrimidines 1b, c in this reaction apart from formylation undergo recyclization into 5-aryl-1,2,4-triazolo[1,5- a]pyrimidin-6-ylmethane derivatives 4b, c, 5b, c, and 6. The structure of the synthesized compounds was determined on the basis of NMR, IR, and MS spectroscopic data and confirmed by the X-ray analysis of the 6-(ethoxy-phenyl-methyl)-5-phenyl-[1,2,4]triazolo[1,5- a]pyrimidine 6, 5-phenyl-6-(1-phenyl-vinyl)-[1,2,4]triazolo[1,5- a]pyrimidine 11, and 7-phenyl-6-(1-phenyl-vinyl)-[1,2,4]triazolo[4,3- a]pyrimidine 12.