Shortcomings in the present standard method of determining the circulating oil concentration in a refrigeration system have led to the current research, wherein a continuous, in-line method of measuring the flowing oil concentration is sought;A literature survey and preliminary property measurements examined properties of oil-refrigerant mixtures that could be measured to infer the oil concentration in the liquid line of a refrigeration system. Four measurement methods were selected for development into oil concentration sensors: a vibrating U-tube densimeter, a new type of in-line viscometer, a prototype acoustic velocity probe, and an optical fiber refractometer;A flow loop capable of simulating a wide variety of liquid-line conditions was constructed for the testing and calibration of the oil concentration sensors. Performance tests of the densimeter, viscometer, and acoustic velocity sensor were conducted over an oil concentration range of 0 to 30 weight-percent for 150 SUS naphthenic oil/R-12, 150 SUS naphthenic oil/R-22, and 150 SUS alkylbenzene oil/R-502 mixtures. The temperatures in the flow loop test section during the performance tests were varied from 70 to 120 F and the pressure was maintained to provide approximately 3 F subcooling. Performance testing of the refractometer was not completed because of severe probe temperature sensitivity and poor repeatability;The performance test results were statistically analyzed to determine the oil concentration measurement uncertainty. The three sensors tested were found to attain the desired ±1 weight-percent uncertainty under a variety of conditions. Application guidelines are presented for the use of the densimeter, viscometer, and acoustic velocity as oil concentration sensors.