The methods of Constantinou and Gani, Ambrose, Joback, Fedors, and Klincewicz were used to estimate the critical temperatures and pressures of the pure components present during the synthesis of biodiesel from soybean oil using supercritical methanol, and the results obtained were compared. Based on the critical properties calculated for the pure components, the pseudocritical and true critical properties of the reaction mixtures obtained in different experimental reaction conditions [temperature, reaction time and methanol-to-oil molar ratio ranges of 523-623 K (8-43 MPa), 15-90 min and 13:1-53:1, respectively] were estimated by means of Kay's rule and the modified Prausnitz and Gunn combination, and by Li's method, respectively. The estimated data of critical properties of the reaction mixtures appearing during the synthesis of biodiesel in supercritical methanol will be very interesting for the calculation of other thermodynamic properties, including the phase behavior of the medium. (C) 2013 Elsevier B.V. All rights reserved.