The main aim of the work is to define the critical aspects of the stability of solid K-based catalysts (K-CaO, K-MgO and K-Al2O3) in the transesterification of rapeseed oil by methanol. High activity of these catalysts is often reported, but their stability is recently a matter for discussion. The attention is focused on (i) the explanation of the reason of potassium leaching from the solid K-based catalysts into the liquid phase during the reaction, (ii) a description of the role of leached K-species in the reaction and (iii) the suggestion of the mechanism of the potassium leaching from the solid K-based catalysts. It was found that the leaching of K-species is caused by glycerol that is formed during the reaction. The mechanism of K-leaching from the solid catalyst was suggested. The strength of alcohols to attack the K-species on solid catalysts increased with the number of alcohol groups and their mutual distances and has following order: glycerol > 1,3-propanediol > 1,2-propanediol > ethylene glycol > methanol. (C) 2013 Elsevier Ltd. All rights reserved.