Abstract Detailed knowledge of refrigerant absorption in refrigerant–oil mixtures is essential for understanding the behavior of hermetic compressors under transient conditions. In some types of refrigeration systems, important operation parameters, such as the equalizing pressure and the energy consumption, are directly influenced by the rates of refrigerant absorption in the compressor oil. In this work, we study the absorption of isobutane (R-600a) through the free surface of a stagnant layer of alkylbenzene lubricant oil in a closed system (PVT cell). Experiments were conducted in a transparent test section that enabled the absorption process to be visually monitored. The mass of isobutane that entered the test section was measured for each experimental test. The instantaneous system pressure and temperatures in the vapor, liquid and interface regions were recorded at temperatures of 25, 40, 55 and 70 °C (nominal). The experimental data were then used to validate a transient mass diffusion model that takes into account the departure from the ideal solution behavior.