This paper deals with the effect of water and catalyst (KOH) amount on the quantity and quality of transesterification products of rapeseed oil by methanol, the methyl ester phase (i.e. yield, conversion), and the side-product, the glycerol phase (i.e. density, viscosity, the mass fraction of glycerol, esters, soaps). The dependencies were described by statistical models. The transesterification was carried out at constant reaction conditions (90 min reaction time, 400 rpm, 60 degrees C). Twelve experiments with the independent factors, amount Of potassium hydroxide (0.65-0.9 mg per gram of oil) and total amount of water (0.24-1.42 mg per gram of oil) naturally present in the reaction components or formed by the neutralisation reaction of free fatty acids and of added water. The data were analyzed by linear regression with respect to regression triplet (complex critical analysis of the model, data and regression method). The analysis resulted in a set of linear and/or quadratic models consisting of statistically proven terms at a statistical significance level of 0.05 and demonstrated that ester in the glycerol phase increases with increasing amount of soaps. (C) 2009 Elsevier Ltd. All rights reserved.