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An important design objective for an automotive air-conditioning system is ensuring sufficient oil return to the compressor. Maintaining proper lubrication of the compressor extends compressor life and thus minimizes warranty and consumer replacement costs. An automotive air-conditioning test stand capable of monitoring transient system parameters was utilized. Real-time oil concentration data were obtained using an optical, oil-concentration sensor installed in the liquid line. The mass flow rate of the refrigerant-oil mixture and the evaporator superheat were varied with an electronic expansion valve. Steady-state oil concentration data are presented for both saturated and superheated evaporator exit conditions. Experimental oil concentration data taken during system startup and quasi-steady compressor-clutch-cycling conditions are also presented. Steady-state results show that oil return rate is maximized by maintaining a two-phase condition at the evaporator exit.