The apparent melting temperatures for a series of polymethylene copolymers were determined as a function of crystallization temperature and level of crystallinity. The copolymers contained from 1·2 to 6·4 mole per cent of either methyl or n-propyl side groups arranged in random sequence distribution. The data could be analysed to yield extrapolated equilibrium melting temperatures by assuming the formation and melting of finite size crystallites. The size of the crystallites is taken to be dependent solely on the crystallization temperature. In accord with the previous observations of Richardson, Flory and Jackson, based on directly observed melting temperatures, it is concluded that at equilibrium the crystalline phase remains pure for n-propyl side group copolymers while solid-solution occurs in the methyl copolymers.