The problem of finding the conditions for homogeneous operation of supercritical chemical reactors is studied experimentally using an isochoric method. Several reacting mixtures related to the transesterification of vegetable oils with supercritical ethanol are studied. The isochoric method provides an indirect way of determining the conditions of phase transitions from the pressure vs. temperature variation of the isochoric lines. The linear variation found both in the homogeneous and the vapor-liquid region of the reacting mixtures allows a clear determination of the transition points at different mixture densities and compositions. The linearity in the vapor-liquid region is also verified by flash computations of the reaction products mixture. The procedure can also be applied to the selection of cosolvents needed to carry out reactions under homogeneous conditions. For this purpose the binaries of methyl oleate, n-octadecane and naphthalene with methanol are also investigated. The colinearity of isochores at the criocondentherm point that has been reported in previous works was also found in the present study. (C) 2010 Elsevier B.V. All rights reserved.