The high content of free fatty acids (FFA) present in non-edible oils and in waste cooking oil for biodiesel production can be esterified with solid acid catalysts. The rate of the esterification reaction is related to the catalyst surface acidity. Thus a fast and accurate method for determining the solids acidity is necessary to prevent low FFA esterification yields. In this paper, the acid sites on the solid catalysts (SiO2 center dot HF and ZnO/SiO2) have been measured by the aqueous method using NaOH as titrating agent. A straight relation between the catalyst esterification and the acid sites measured by this method is found. This method is fast and easy to perform in almost all research laboratories and can provide an inexpensive way of selecting a catalyst for improved yield in the FFA esterification process. (C) 2013 Elsevier Ltd. All rights reserved.