The influence of the molar mass and chemical composition of copolymers on the thermodynamics of the micellization process has been investigated using two polystyrene-block-poly(ethylene/propylene) copolymers with the same polystyrene (PS) block length but different poly(ethylene/propylene) lengths (PEP). Micelle formation was studied in n-octane and 5-methylhexan-2-one, selective solvents for PEP and PS blocks, respectively. Light-scattering measurements were carried out in order to determine the critical micelle temperature, T_cm, of the different micelle solutions. From T_cm and concentration data, the standard Gibbs energy, standard enthalpy and standard entropy of micellization were estimated. The characteristics of the copolymer and the block location in the micelle structure were found to influence the thermodynamics of the micellization process. Anomalous behaviour was observed for one copolymer in 5-methylhexan-2-one.