The aim of the present study is to suggest two new fluids as alternatives of R12 in a vapour compression cycle. These fluids are the methyl trifluoromethyl ether (E143a) and the hexafluorocyclopropane (C216), which belong to halogenated derivatives of dimethyl ether and cyclopropane, respectively. The two suggested fluids have low degree of toxicity and a quite short atmospheric life. They also have suitable boiling points and a relatively simple molecular structure. A method of estimation of the thermodynamic properties of E143a and C216 is presented, The theoretical performance of the new fluids are evaluated and compared to those of the R12.