The apparent molar heat capacities C(p,phi), and densities of adenine, 2-aminopurine, 9-methyladenine, 2,2-dimethylaminopurine, 9-ethylguanine, 2-methylamino-9-methylpurine, 2-dimethylamino-9-methylpurine, 9-methyl-2-butyladenine, and 6,9-dimethyl-8-butyladenine were determined experimentally using a differential adiabatic scanning microcalorimeter DASM-1M and an Anton Paar DMA 60/602 digital densimeter. In the series of alkylated adenines and 2-aminopurines, a linear dependence was found between the partial molar heat capacities C(p,2)theta and the number of substituted methylene groups n(CH2) belonging to the skeleton of the molecule. The experimental data of the apparent molar heat capacities of 9-methyl-2-alkyladenines and 6,9-dimethyl-8-alkyladenines indicated erratic changes in DELTAC(p,2)THETA with elongation of the n-alkyl chain.