Alkylation of 3-alkyl-6-methyl-5,7-dioxo-4,5,6,7-tetrahydro-1,2,3-triazolo[4,5-d]pyrimidines I with alkyl halides in dimethylformamide in the presence of potassium carbonate, or by analogous alkylation of sodium salts of compounds I afforded the respective 4-alkyl derivatives III but not 5-alkyloxy-3-alkyl-6-methyl-7-oxo-6,7-dihydro-1,2,3-triazolo[4,5-d]pyrimidines IV or isomeric 1,3-dialkyl-6-methyl-5,7-dioxo-4,5,6,7-tetrahydro-1,2,3-triazolo[4,5-d]pyrimidinium halides II.