Dynamic light scattering and viscometry measurements have been carried out on three AB diblock copolymers in water/propan-2-ol mixtures. The A moieties in each case were poly(2-vinylpyridine-1-oxide) for which both water and propan-2-ol are good solvents. The B Moieties were either polystyrene or polydimethylsiloxane, for which both water and propan-2-ol are poor solvents, but water is the worse solvent. In all cases, the hydrodynamic size of the micelles (assumed spherical) was found to increase with increasing water content in the solvent mixture; the aggregation number was also found to increase. The former finding is in agreement with the theoretical predictions of Munch and Gast, but not the latter.