Three models that predict the performance of an oil-flooded single-screw refrigerant compressor are presented. The thermodynamic equations which are a basis for all the models are derived using a control volume approach. The mechanisms for power input to the oil are presented and three of these mechanisms, oil shear, oil displacement, and external oil pumping, are dealt with analytically. The computer model, the most comprehensive model developed, gave detailed thermodynamic information about the internal processes. For the example presented, the powers calculated included (1) the shaft power (99.9 kw), (2) the preheat power (14.8 kw), (3) the closed compression power (85.1 kw), (4) the oil shear power (8.1 kw), (5) the oil displacement power (4.3 kw) and (6) the external oil pumping power (0.85 kw). For this case, the model also predicted the contributions to volumetric inefficiency (8.7%) from preheat (3.75%), recirculating oil (3 .3%) and refrigerant leakage (1.65%). It also gave the overall compression efficiency (71.5%) and the closed compression efficiency (87 .4%) along with the internal pressures, temperatures, and oil flow rates during the compression process. The star tooth tip leakage path was the site of the most oil leakage contributing 2.6% to the volumetric inefficiency. The preheat phenomenon gave rise to extra power consumption during the closed compression process (1/2% increase in ideal compression power for each 1°C additional preheat). The eight variables necessary to define the geometry of a single screw mechanism are given along Awith two techniques to determine the pocket volume and the main rotor shear area. / M. S.