Four diblock copolymers of styrene oxide and ethylene oxide were prepared by oxyanionic polymerization: E45S10, S15E63, S17E65, and S20E67. Surface tension measurements were used to determine critical micelle concentrations of copolymer E45S10 in aqueous solution at several temperatures and thereby its standard enthalpy of micellization. A compilation of known results for copolymers with hydrophobic blocks formed from either styrene oxide or 1,2-butylene oxide shows a discontinuity at S5 or B10 in the block length dependence of the Gibbs energy of micellization which is assigned to lengthy hydrophobic blocks being tightly coiled in the molecular state in water. Light scattering was used to determine micellar association numbers and radii, and scaling exponents were established for the dependence of these parameters on S-block length. Phase diagrams defining regions of hard and soft gel were determined by tube inversion and Couette rheometry.