We present an approximate analytical analysis of the release of catanionic mixtures from gels. The starting points are the monomer-mixed micelle equilibrium, described by using regular solution theory, and the one-dimensional diffusion equation. Focusing on a half-infinite planar system, we first point out an exact reduction of the problem to a system of ordinary differential equations. By using the pseudo-steady-state approximation and the integral-balance method, we then derive a single nonlinear equation for the mole fraction of drug in micelles at the extraction front. This equation may be readily solved numerically (or graphically), and once the solution is found, all quantities of interest may be determined in closed form. Comparisons with numerical solutions of the fully nonlinear problem indicate that the errors resulting from the approximations typically do not exceed 10%. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 1402-1408, 2011