The exothermic conversion of 1,3-dimethyl-4-alkylamino-5-nitrosouracils into theophylline or its 8-substituted derivatives was investigated. The conversion was carried out in the solid phase. The course of this reaction is affected by steric and electronic properties of the substituents present in proximity to the reaction centre of the starting substance. The values of molar reaction enthalpies (Delta H-T) of 19 derivatives of 1,3-dimethyl-4-alkylamino-5-nitrosouracils were determined. The relationship between these values and the structure of the substituent was examined.