The vapour compression cycle is the most common type of refrigeration cycle in use today. Most vapour compression systems are simple, having only four major components: a compressor, a condenser, an expansion device and an evaporator. Multistage vapour compression systems are more complex with, for example, extra compressors, aftercoolers, intercoolers, flash tanks and liquid-to-suction heat exchangers. The study performed here considers 121 different configurations operating at condensing and evaporating temperatures that range from -50 to 50 degreesC. The refrigerants used are ammonia, R-22, R-134a, R-152a and R-123. The basis of comparison for the systems is multistage effectiveness. Multistage effectiveness is a novel term defined as the ratio of the coefficient of performance of a multistage system to the collective coefficient of performance of an equivalent group of basic single-stage systems operating at the same cooling capacities and evaporating and condensing temperatures. Equivalency here is defined on the basis of achieving the same cooling capacity at their respective temperatures as dictated by the multistage systems. The vapour compression system model presented here was put through genetic optimization with interesting results. Copyright (C) 2001 John Wiley & Sons, Ltd.