Catalyst-free transesterification of leather tanning waste with high free fatty acid (FFA) content at supercritical condition was reported in this work. The experiments were performed in batch system at various temperatures (250-325 degrees C) under constant pressure of 12 MPa and methanol/fatty oil molar ratio of 40:1 for reaction time of 2-10 min. Kinetic modeling of formation of fatty acid methyl esters (FAMEs) that incorporate reversible esterification and non-reversible transesterification simultaneously was verified. The proposed semi-empirical model was fitted against kinetic experimental data over temperature range studied. The kinetic parameters (i.e. k'(TE), k'(E), and k(E)') were determined by nonlinear regression fitting. Thermodynamic activation parameters of the reactions were evaluated based on activation complex theory (ACT) and the following results are obtained: Delta G(double dagger) >0, Delta H-double dagger>0, and Delta S-double dagger<0. The activation energy (E-alpha) of transesterification, forward and reverse esterification reactions was 36.01 kJ/mol, 28.38 kJ/mol, and 5.66 kJ/mol, respectively. (C) 2012 Elsevier B.V. All rights reserved.