In general, two temperature levels are required in supermarkets for chilled and frozen products; temperatures of around +3°C and -18°C are commonly maintained respectively. In such application, with a large difference between evaporating and condensing temperatures, the cascade or other two-stage solutions become favorable and are adaptable for the two temperature level requirements of the supermarket. The use of CO2 in supermarket refrigeration started with applying CO2 as the working fluid in indirect systems for freezing applications. Then it has been applied in cascade solutions mainly with R404A in the high temperature stage. After gaining experience and the availability of components CO2 has been used in trans-critical system solutions, mainly in northern Europe. The two main trans-critical system solutions applied are the parallel and the booster, the later is the latest in CO2 systems development series and has been applied in most of the installations in Sweden. CO2 trans-critical booster system is the main system that has been investigated in this research work. A simple schematic of this system is shown in Figure 1. Two temperature levels are used to maintain the quality of fresh and frozen products. The absorbed heat rejected in a gas cooler/condenser. In case of demand for space /tap water heating, part of or the entire rejected heat can be recovered in a de-superheater.