Block copolymers were prepared by copolymerization of ethylene oxide and butylene oxide to have identical overall composition and chain length but to differ in their distributions of oxyethylene (E) block length by virtue of the sequence of polymerization of the two monomers : ethylene oxide first to obtain copolymer E41B8, and butylene oxide first to obtain B8E11. Gel permeation chromatography was used to show that the chain length distributions were in accord with the prediction of the kinetic scheme of Weibull and Nycander. The effect of E-block-length distribution on the self-association (micellization) of the two copolymers in water was determined by means of surface tension and light scattering (dynamic and static) measurements. Compared with copolymer E41B8 under similar conditions, it was found that copolymer B8E11 micellized more readily and formed larger micelles. The effect of the difference in micellar properties on the gelation of moderately concentrated solutions of the two copolymers is described and discussed.