This paper presents the results of a search for optimum working fluids appropriate for solar cooling of buildings. The Underwriter's safety requirements, system operating reliability and fluid thermal stability were the primary factors used in the screening process. Although minimum development problems would appear to exist when using steam, a low power level, multistage turbine, having a high degree of superheat, would run at excessive shaft speed resulting in high turbine disc stresses. In addition, the machine would be delicate, expensive, and require tight tolerances thereby complicating its manufacture. Moreover, the presence of two-phase flow in its bearings would cause erratic operation. Water is therefore considered to be an unsuitable fluid for such systems. Among the various organic fluids considered, including those proposed for various prototype developments presently in progress, only the halocarbon compounds R-11, R-113 and R-114 and the fluorinert compounds FC-75 and FC-88 satisfied safety requirements. Their two-phase pressure drops per unit length, mean two-phase heat transfer coefficients and maximum allowable heat flux were determined for Reynolds numbers of 10,000 to 120,000 at an average, fully-developed, liquid-phase velocity of 1.22 m/s, and 16.6 mm inside tube diameter. A preliminary assessment based on these results, and those available of static, thermal stability tests together with the values of the vapor pressures in the condensing temperature range and the boiling points is presented. It indicates that R-113 followed by FC-88 are the most suitable fluids for such application. A final assessment however, should be based on dynamic thermal stability tests, where boiling, condensing and other stresses are imposed on the fluids.