An inverse problems method is applied to a two-phase liquid-liquid system in a rotating disc contactor (RDC). The dispersed phase is modeled by population balance equations, which are solved by a Monte Carlo method, together with the equations for the parametric derivatives of the solution with respect to the parameters of the model. The best-fitting problem is solved by a gradient search method. Because the inverse problem is ill-posed, the iteration procedure is augmented by an appropriate termination criterion to stabilize the calculations. The parametric derivatives of the solution can be used to quantify the relative importance of different parameters of the model. It is shown that the model's parameters, which are identified on one set of the experimental data, adequately describe the behavior of the system under another unfitted operation condition, that is, the proposed method can be applied to scale-up problems. (c) 2005 American Institute of Chemical Engineers