The precipitation of calcium oxalate monohydrate (COM) as a waterproofing material for concrete constructions is a promising approach. The kinetics of spontaneous precipitation of the calcium oxalate from supersaturated solutions in batch reactors was investigated. The precipitation took place in supersaturated solutions of equal concentrated solutions of sodium oxalate and calcium nitrate and was monitored when COM precipitated from the solutions as a function of time. The experiments were done both in unseeded and in solutions seeded with COM crystals and also with concrete grains (Portland cement). COM formed exclusively in all cases. From the analysis of the kinetics data, it was concluded that the prevalent mechanism for COM precipitation was surface diffusion. Satisfactory fit of the data to the polynuclear model yielded values for the surface energy ca. 30 mJ m(-2). The relatively low values obtained suggested mainly heterogeneous nucleation. The rates of COM precipitation increased with increasing amounts of inoculating COM seed crystals suggesting significant contribution of seeded growth in the overall process. Inoculation with concrete grains resulted in lower rates possibly due to the release of inhibiting species in the supersaturated solutions from the partially dissolving substrate. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.