The refrigeration and heat pump systems are lately characterized by a remarkable evolution period. The principal reasons that have determined changes are the substitution of environmentally unfriendly refrigerants and the energy saving necessity. As fundamental component of a vapour compression plant, the compressor is an object of optimizations, the variation of the compressor speed, obtained regulating the Supply Current frequency of the compressor motor, allows to obtain energy savings. The principal aim of this paper is the determination of an experimental model that represents the variable speed reciprocating compressor working. In particular, equations that allow to get the refrigerant mass flow rate, the compressor input power and the cooling capacity in terms only of the frequency are obtained. The experimental model allows to determine the optimum frequency for each working condition and then the related energy saving. Copyright (C) 2008 John Wiley & Sons, Ltd.