The present work is related to the simulation of a reciprocating hermetic compressor operating in transient regime. Hermetic compressors consist of a motor-compressor assembly hermetically sealed in a welded steel shell. Main components are: electric motor, suction and discharge mufflers, discharge line and the compressor itself, formed by the suction and discharge chambers, the pistondriving mechanism assembly and the cylinder body. The model here presented extends existing thermodynamic steady-state models for the transient operational condition. The system is divided into a number of control volumes, for which homogeneously distributed properties is assumed and fundamental conservation equations are applied. Special attention is given for foam formation, during startup. In such conditions, liquid oil can be drawn into the cylinder, causing a deterioration in the performance. Information on the phenomenon is scarce, which led to an effort of reproducing it at laboratory conditions. A saturated mixture of R134a and polyolester oil, at a given pressure, was submitted to controlled depressurization. The foam formation process (rate, height and bubble size) was measured with the help of a digital video camera. A number of runs were carried out, for different pressure drop rates and initial refrigerant concentrations. The experimental data was employed to adapt an existing model of foam formation. A new function for the volume rate of coalescence of gas bubbles was empirically determined. Computational Fluid Dynamics techniques were also used to predict the turbulent three-dimensional flow of refrigerant in the shell side.