The separation of chromium(VI) from industrial wastewaters is of great interest mostly due to the high toxicity of this metal. Although the extraction of Cr(VI) with anionic extractants (and among them, quaternary ammonium salts) has been widely studied using different technologies, a suitable equilibrium model it has not been reported due to the complex behavior of the organic phase. In this work a general methodology for modeling of the chemical equilibrium of Cr(VI) is presented, Discrimination between mathematical models that take account of the nonideal behavior of the organic phase led to the selection of a one-parameter empirical function of the total extractant concentration, K = K(c)c(0)(n). The parameter values estimated for this model are the equilibrium constant (Kp = 0.197) and the fitting parameter (n = 0.661), and they are used in the expression for the description of the extraction equilibrium : 0.197 = [Cl-](2)[(AQ)(2)CrO4]/[CrO42-][(AQ)Cl](2)c(0)(0.66) This equation can satisfactorily describe the experimental behavior of chromium(VI) extraction with Aliquat 336 and thus provides a useful expression that could be introduced into mathematical models directed toward the design and optimization of the separation process.