Abstract: To control the inner diameter change of double emulsions and to improve the surface finish of the resulting poly(-methyl styrene) (PAMS) capsules, the effects of electrolyte in the inner and outer water phases on the water migration and phase separation behaviors in the oil phase layer during curing process were investigated. The results show that addition of electrolyte only in the inner water phase leads to a migration of the water from the inner to the outer water phase and thus results in a shrinking of the double emulsions, potentially wrinkling the resulting PAMS capsules. When adding electrolyte into the inner and outer water phases at the same time, the chemical potential of the inner water phase balances with that of the outer water phase, so that the generation of water microdroplets and the migration of water in the oil phase layer during the curing process are suppressed and then the vacuoles in the shell wall of the resulting PAMS capsules are reduced. Moreover, through adjusting the concentrations of electrolyte in the inner and outer water phases, the densities of the three phases of the double emulsions can also be matched for improving the sphericity and wall thickness uniformity of the resulting PAMS capsules.