The B3LYP/6-31G** quantum chemical method was used to study the mechanism of the oxidative alkylamination of diazinones taking 1,3,6-trimethylpyrimido[4,5-d]pyrimidine-2,4,7(1H,3H,6H)-trione (lumazinone) as an example. The first limiting step of this reaction included a nucleophilic addition of alkylamine with the formation of the σH-adduct, which was oxidized by potassium permanganate in the second step. The alkylamination step proceeded in the associates formed by the lumazinone dimer and several alkylamine molecules involved in both the nucleophilic attack and the bifunctional catalysis of the process. The correlation of the Parr indices with the experimental data was studied for a number of diazinones.