New catalysts and environmentally benign processes may lead to methyl ester production with improved properties at competitive costs. In this study. transesterification of waste frying oil to biodiesel using tetramethylguanidine as a strong base catalyst was conducted. The influence of catalyst concentration and of certain physicochemical properties of waste frying oil was investigated. Experiments were also performed on a semi-refined cottonseed oil for comparison purposes. Experimental results showed that methyl ester conversion was dependent on the type of oil, catalyst concentration and reaction time. (C) 2011 Elsevier Ltd. All rights reserved.