Crystallisation processes from artificially prepared over-saturated solutions were investigated in order to develop a new technology for modifying the permeability of rock or soil formations. The present work reports the results of experimental and numerical studies of the effect of precipitation inhibitors on the crystallization of gypsum from over-saturated solutions. Without precipitation inhibitor gypsum crystallisation takes place shortly after mixing of sulphate and calcium containing solutions if the solubility is exceeded. Precipitation inhibitors are able to prevent spontaneous crystallisation temporarily. The main effect of the inhibitor is to increase the germination time (induction or retardation time), resulting in a temporal stability of the solution. Having a temporarily stable over-saturated solution allows the injection of the solution into the porous matrix without precipitation of the mineral, which later crystallized inside the matrix. Numerical simulations of the effect of precipitation inhibitors on the crystallization process are carried out at two different levels of analysis: at the scale of the crystal seeds (referred here as the micro-scale) and also a macro analysis of the crystallization process of an aqueous over-saturated solution, which is kept continuously in motion (stirring experiment). The micro analysis is considered in order to define a constitutive equation that relates the effect of the inhibitor upon the saturation index of the mixture, to be used in the macro simulation of the stirring experiments. The developed numerical code can be used to simulate crystal formation and growth from natural and artificial solutions. The present numerical study has revealed several new features of the effect of precipitation inhibitors on the crystallization process, which are difficult to observe experimentally. (c) 2006 Elsevier B.V. All rights reserved.