In the paper the nonlinear Kedem-Katchalsky model equations of the volume flux in a double-membrane system are presented. The starting point are the classic Kedem-Katchalsky equations. The model, which basis is the equation alpha(Jv)2 + betaJv - gamma = 0,, describes the stationary volume transport in the double-membrane system. The equations are essential in order to calculate the volume flux value (Jv) in the dependence on stimuli (i.e. the pressure difference (deltaP) and solutions concentrations (Cl, Cr)), and on the membranes parameters (Lp, omega, sigma). In the paper, research were carried out upon the influence of the pressure difference (deltaP) and solutions concentrations (Cl, Cr) on the volume flux value (Jv).