This thesis is mainly focused on the reactions of oxo-pyrimidines and oxo-imidazoles with nitric acid in sulfuric acid and properties of the gem-dinitro products formed in this process. Low temperature nitrations of 2-methylimidazoles produced – in addition to the known 2-methyl-5(4)-nitroimidazole – 2-(dinitromethylene)-5,5-dinitro-4-imidazolidinone and parabanic acid. This tetranitro compound was also obtained via nitration of 2-methyl-4,4-dihydro-(1H)-5- imidazolone. Thermal decomposition of 2-(dinitromethylene)-5,5-dinitro-4-imidazolidinone yielded 2-(dinitromethylene)-4,5-imidazolidinedione, which also was the product from the nitration of the new compound 2-methoxy-2-methyl-4,5-imidazolidienedione. Treatment of 2- (dinitromethylene)-5,5-dinitro-4-imidazolidinone with aqueous ammonia resulted in the previously unknown 1,1-diamino-2,2-dinitroethylene (Paper I). The nitration of some 2-substituted pyrimidine-4,6-diones in sulfuric acid, which afforded previously unknown 5,5-gem-dinitro-pyrimidine-4,6-diones in high yields, was studied. Alloxane was prepared in a one-step procedure by thermal decomposition of 5,5-dinitrobarbituric acid in hot acetic acid. The gem-dinitro products were found to be easily attacked by nucleophiles with concomitant formation of gem-dinitroacetyl derivatives, which in turn could be further hydrolysed to salts of dinitromethane and triureas (Papers II and III). Nitration of 4,6-dihydroxypyrimidine in sulfuric acid yielded nitroform as the sole product. This behaviour was tentatively explained by the formation of an intermediate, 5,5-dinitro-4,6- dihydroxypyrimidine, which underwent hydrolysis in the nitrating acid into gem-dinitroacetyl formamidine. This compound was further nitrated in the same reaction mixture into trinitroacetylformamidine, which finally underwent hydrolytic cleavage into nitroform. It was also demonstrated that gem-dinitroacetylureas could produce nitroform upon nitration. The structures of the proposed trinitroacetylureas were confirmed by the isolation of one of their derivatives (Paper IV). / QC 20100907