Interactions of 5,6-dimethyl- (1), 3,5,6-trimethylthieno[2,3-d]pyrimidin-4(3H)-ones (2) and 2,3-dimethyl- (5-7), 2-methyl-3-methoxycarbonylthieno[2,3-d]dihydropyrrolo-, - tetrahydropyrido-, tetrahydroazepino[1,2-a]pyrimidin-4-ones (14-16) with nitrating mixture were investigated. For the first time it is shown, that in dependence on the presence of substituent in position 2 and 3 of pyrimidine and thiophene rings reaction goes in various directions; by electrophilic ipso-substitution of methyl groups at C-5 by nitro group, or its oxidation up to carboxyl groups with formation corresponding 5-carboxy derivatives. It is revealed, that at absence of the substituent in position 3 (compound 1) the electrophilic ipso-substitution of methyl group by nitro group with formation of 5-nitro derivative took place. It is found, that at interaction of compounds 2,5-7, 14-16 with nitrating mixture instead of substitution of methyl groups at C-2, goes in an unexpected direction, i.e. there are oxidation of methyl groups or electrophilic ipso-substitution of methoxycarbonyl groups in position 3 by nitro group.