In investigating the growth kinetics of heteroepitaxial films in situ by force microscopy, we have observed several instances where the lateral force contrast on the growing film exhibits a strong dependence on the time of exposure to the growth solution and on the driving force for growth (solute concentration). We present results for three epitaxial growth systems in aqueous solutions: CaC₂O₄ on CaCO₃, CaSO₃ on CaCO₃, and PbSO₄ on BaSO₄. Chemically and topographically identical regions of the growth at higher concentrations exhibit higher friction, and this contrast decreases with time of exposure to the solution. These observations suggest that epitaxial growth occurs by a fast condensation step incorporating a high defect density, followed by a slower densification process in which defects are annealed out by the action of solvent and/or absorbed ions.