A systematic investigation of the effects of polymer chemical composition and molecular mass on (i) the micellization of polyoxyalkylene block copolymers [poly(ethylene oxide)/poly(propylene oxide), PEO/PPO, and poly(ethylene oxide)/poly(butylene oxide), PEO/PBO] in an organic solvent(p-xylene : selective solvent for the PPO and PBO blocks) in the presence of some water and (ii) the solubilization of water (selective solvent for the PEO blocks) in these systems is presented. The ability of the ("reverse") micelles to solubilize water has been employed for the detection of the critical micellization concentration ("reverse" cmc). The "reverse" cmc values were in the range 0.01-0.1 m (5-20 wt %), while the maximum water solubilization values were 0.3-0.5 g of water per g of copolymer. The cmc decreased with increasing copolymer molecular mass for a given copolymer chemical composition. A notable increase (more than doubling) in the water solubilization capacity (expressed in water molecules per EO segment) was observed when the PEO content of PEO-PPO-PEO copolymers decreased from 40 to 20%. A simple model for the reverse micelles is developed (based on the volume fractions of the different components and the effective length of a copolymer molecule) that captures experimental trends and allows the estimation of the (spherical) micelle radius and aggregation number.