Abstract Crystal growth has been promoted in the fluid cell of an Atomic Force Microscope (AFM) by passing Ba–Sr–SO 4 aqueous solutions over barite (0 0 1) cleavage surfaces. Steps advance in structural continuity with the original barite (0 0 1) surfaces and two-dimensional nucleation occurs preferentially on the newly-formed terraces. The terraces are, on average, 7.5% lower than pure barite terraces. Since the ionic radius of Sr 2+ is smaller that the ionic radius of Ba 2+, the reduction of terrace height is consistent with an extensive incorporation of Sr 2+ into the barite structure. Therefore, it can be considered that the newly-formed terraces have compositions corresponding to terms of the Ba x Sr 1− x SO 4 solid solution. A non-linear dependence of step rate on [SrSO 4] concentration in the solution (and therefore on supersaturation) has been found. The growth behaviour has been discussed by considering both the physicochemical properties of the Ba x Sr 1− x SO 4 solid solution–aqueous solution (SS–AS) system and a kinetic-based step growth model.