A Taguchi orthogonal array was used to investigate the effects of catalyst type, catalyst concentration, temperature and methanol to oil molar ratio on the heterogeneous esterification of free fatty acids (FFA) in waste cooking oil (WCO) using ion-exchange resins. Analysis of Variance (ANOVA) was also applied to study the final FFA conversion in response to the investigated factors. The F-test indicated that catalyst type and methanol: oil molar ratio significantly influenced the FFA conversion. The optimal parameters of FFA esterification in WCO using ion-exchange resins obtained within the experimental ranges studied are as follows: Catalyst Amberlyst-15 at 4 wt.%, temperature of 65 degrees C and a methanol to oil molar ratio of 15:1. Under these conditions, a maximum FFA conversion of 60.2% could be obtained. The kinetics of the optimised FFA esterification catalysed by 4 wt.% Amberlyst-15 fitted well to a Langmuir-Hinshelwood (L-H) based model. (C) 2012 Elsevier B.V. All rights reserved.