The refrigerant mostly use until now (CFC, HCFC and HFC refrigerant) has a negative influence on the environment and therefore it is wanted to find alternative refrigerant without any negative influence on the environment. A change to natural refrigerant like Carbon Dioxide (CO2) is therefore of benefit of the environment. The main challenge for CO2 based refrigerant systems is to increase the performance of the heat exchangers. Especially there is a need for information concerning heat transfer and pressure drop in evaporator and condenser with CO2 as refrigerant. The reason this is the very high reduced pressure with CO2 compared to the reduced pressure using CFC, HCFC and HFC. CO2 has greater heat conductivity than CFC, HCFC and HFC so the influence of oil in the refrigeration system is expected to have a greater influence on the system performance. The main result of the project described in the present report is measured heat transfer coefficient and pressure drop for flow boiling of oil free and oil contaminated CO2. Measurements have been done on tube with internal diameter of 10 mm and 4 mm- The mass flux has been varied from 90 kg/m2s to 750 kg/m2s, heat flux from 5 kW/m2 to 40 kW/m2 and evaporation temperature from -10°C to -35°C corresponding to a reduced pressure between0.35 and .16. The conclusion from the measurements is that oil gives a reduction in heat transfer coefficient compared to pure CO2. The reduction in heat transfer coefficient is greater at high qualities than at low qualities even at low oil concentration in the circulation oil/CO2 mixture. The reduction of the heat transfer coefficient with increasing oil concentration is lower at high qualities than at low qualities.