The heat capacities and enthalpies of phase transitions for urea and 13 alkyl derivatives of urea [methyl-, 1,1-dimethyl-, 1,3-dimethyl-, ethyl-, (1-methylethyl)-, tetramethyl-, 1,1-diethyl-, 1,3-diethyl-, butyl-, (1,1-dimethylethyl)-, 1,3-bis(1,1-dimethylethyl)-, tetraethyl-, octyl-, and (1-methylpropyl)urea] in the temperature range from 5 to 320 K have been measured by adiabatic calorimetry. The thermodynamic functions of these compounds were obtained from smoothed heat capacity values. The results were used to show that an additive scheme for the calculation of the thermodynamic properties of crystals of alkyl derivatives of urea, without taking into consideration the phase distinctions in crystals, gives reasonable accuracy.