The influence of (mono) ethylene glycol (MEG) on polymorphism and the resulting morphology of calcium carbonate have been Studied for activity-based supersaturation ratios in the range of 3-10 and temperatures from 25-80 degrees C in mixed solvents of ethylene glycol and water at ratios of 0-90 wt%. The presence of a co-solvent in the solution affects the supersaturation, because of changes in the activity coefficients and the solubility of the salt, a fact that is Usually not accounted for in similar studies in the literature. In the present study, the effect of the solvent was isolated from the accompanying change in the supersaturation. MEG was found to affect both the polymorphic abundance in the precipitates, the morphology of the particles and the transformation rates. High concentrations of MEG favoured the precipitation of vaterite and higher temperatures promoted the formation of aragonite. The particle size was reduced in experiments with high MEG concentrations at supersaturations ratios comparable to water solutions, illustrating that the nucleation rate is affected by the co-solvent. The morphology of the calcium carbonate particles was changed at various conditions of MEG concentrations and temperatures from cubes of calcite to spherical, flower-like and dumbbell particles of vaterite and aragonite needles. MEG prolongs the transformation time of metastable polymorphs and the effect was shown to be caused by the solvent itself, probably as a result of kinetic stabilization by delaying the growth rate of the more stable polymorphs. (C) 2009 Elsevier B.V. All rights reserved.