[en] The reactions of various substituted methylpyrimidines and bicyclic 1,2,4-triazolo[1,5-a] pyrimidines containing methyl groups in the pyrimidine ring with substituted benzaldehydes and heterocyclic aldehydes of the pyrazole and furan series have been studied. As a result, a series of styryl- and vinyl pyrimidine derivatives containing conjugated π-bonds were synthesized. In some examples, in particular, in the reaction of 2-hydroxy-4-methyl-6-phenylpyrimidine with para-dimethylamino- and para-diethylaminobenzaldehydes not styryl derivatives were isolated but the addition products of the starting reagents – 2-hydroxy-4-[(2-(4-dialkylamino)-phenyl)-2-hydroxyethyl)]-6-phenylpyrimidines, i.e. the hydration products of the expected styryl derivatives