Barite-celestite solid solutions have been synthesized under controlled conditions by the counter-diffusion of Ba2+, Sr2+ and SO42–ions through a porous transport medium (silica gel), to investigate the factors which control compositionalzoning. The equilibrium compositions of solid solution and aqueous solutionhave been determined from the relative solubilities of barite and celestite, predicting that virtually pure barite should precipitate from Sr-rich solutions. However, nucleation and growth in a porous medium, where mass transport is by diffusion, takes place at very high supersaturations. The threshold supersaturation for nucleation is inversely related to the solubility and is therefore much higher for barite than celestite. The balance between the thermodynamics and kinetics of nucleation and growth at high supersaturations leads to zoning patterns the reverse of those predicted at equilibrium. At high supersaturations the zoning is periodic and sector-controlled with many of the general features observed in natural minerals. Oscillatory zoning with compositional gaps can take place without the need to invoke miscibility gaps or periodic variations in externally controlled intensive parameters.