A computer model for calculating the performamce of a scroll compressor with a self-adjusting back-pressure mechanism was developed. The model gives detailed geometrical and thermodynamic information concerning the internal processes. It calculates the influence of internal leakage between the neighboring gas pockets with its viscous effect. It also calculates the contribution to adiabatic inefficiency from gas flow passing through small apertures located in positions opened to the intermediate compression gas pockets. This paper describes the theory used in the model and then presents some numerical results of p-v diagram, back-pressure, and the adiabatic efficiency of the machine for several operating conditions.