INTRODUCTION Carbon Dioxide (R744) was a well known and widely accepted refrigerant in the early 1900's@ but its popularity reduced with the introduction of fluorocarbons. The revival of R744 as a refrigerant started over a decade ago in Europe with the work of Dr. Gusav Lorentzen and Dr. Jostein Petterson [1]. This sudden rediscovery was invoked by growing environmental concerns of global warming and ozone depletion. R744 has some very attractive properties@ which makes it destined to be used as a working fluid. It is non-flammable@ non-ozone depleting@ has good heat transfer properties@ a high volumetric capacity@ it is easily available and economic. However its critical temperature is 31.1oC@ which is generally lower than the heat rejection temperature of a typical refrigeration and air conditioning system. Thus@ wherever the heat rejection temperature is greater than the critical temperature@ R744 must operate in a transcritical cycle@ i.e@ with a sub critical low-side pressure and a supercritical high side pressure. The work of Dr. Peter Neksa has already proved the advantages of using R744 for water and space heating applications. In the field of automobile cooling systems@ R744 also has proved advantageous over the conventional system in terms of better cooling performance@ improved fuel consumption and zero ozone depletion rate [2]. There is a drive to move R744 towards space cooling and it is being developed and tested across Europe. 1. Man-Hoe Kim@ Jostein Pettersen@ Clark W. Bullard@ 2003@ Fundamental process and system design issues in CO2 vapour compression system. 2. Peter Neksa@ Jostein Petterson and Geir Skaugen@ 2006@ CO2 Refrigeration@ Air Conditioning and Heat Pump technology.