The dependence of block copolymer interdomain spacing d on polymer volume fraction phi(P) in solution, d similar to phi(P)(-phi), is examined through a series of styrene-isoprene diblock copolymers in four solvents, three of them strongly selective (nonsolvents for polystyrene). The exponent beta is found to depend on solvent selectivity (both temperature and chemical identity of solvent; beta increases with selectivity), on mesophase (beta is largest when the insoluble blocks form lamellae, then cylinders, then spheres), and on diblock composition within a given mesophase (beta is largest when the soluble blocks are most highly diluted). Values of beta as large as unity are observed, indicating that selective solvents can swell d quite substantially over the value for the bulk diblock. In one example, a 2.5-fold increase in d is obtained when a cylinder-forming polystyrene-rich diblock is diluted into its mirror-image polystyrene-poor cylindrical mesophase.