The micellization of binary mixtures of water-soluble block copolymers EmBnEm and EmSnEm in dilute solution was investigated by light-scattering methods. We use the notation E, B, and S to denote chain units derived, respectively, from ethylene oxide, butylene oxide, and styrene oxide and the subscripts to denote number-average chain lengths in chain units. Two distinct distributions of micelles were formed in solutions of a 50:50 wt % mixture of copolymers E64B20E64 and E137S18E137, which had hydrophobic blocks of similar length but very different hydrophobicity. One distribution of micelles was formed in solutions of a mixture (50:50 wt %) of copolymers E135B20E135 and E82S9E82, which had hydrophobic blocks of different length but similar hydrophobicity. In this case, the properties of the micelles formed in solutions of the mixture were very similar to those of micelles of E82S9E82 alone. This result extended to concentrated solutions, because the hard-gel boundary for the mixture proved to be very similar to that of solutions of E82S9E82 alone.