A model of a liquid-liquid rotating disc contactor is postulated which takes into account drop breakage but not coalescence, individual drop motion and contact time in a stage, mass transfer coefficients which are affected by contamination, and axial mixing in the continuous phase only. The model is intended to guide industrial design work under conditions of interfacial contamination which are expected in industrial practice: academic work has concerned itself primarily with very clean systems resulting in mass transfer correlations which are not appropriate for industrial design work. The work addresses the question whether mass transfer data obtained experimentally for single drops can be used with confidence in column design. It is shown that average drop sizes and hold-up can be simulated well without using adjustable parameters for each experiment for hold-up not exceeding 20%. The mass transfer performance can be modelled using a contamination factor which varies with drop diameter and residence time; however, the effect of contamination is apparently reduced in the RDC perhaps by virtue of reduced contamination gradient on the drop surface brought about by non-uniform drop motion. The work indicates that mass transfer data for single drops in vertical motion may not be valid for column design (although giving conservative results) unless drops can be shown to be very clean in both cases.