Disclosed are compositions useful in a wide variety of applications, including heat transfer fluids which possess a highly desirable and unexpectedly superior combination of properties, and heat transfer systems and methods based on these fluids. The preferred heat transfer fluid comprises from about 1 to about 40 percent, on a weight basis, of carbon dioxide (CO2) and from about 99 to about 60 percent, on a weight basis, of a compound having the Formula I XCFzR3-z (I), where X is a C2 or a C3 unsaturated, substituted or unsubstituted, alkyl radical, each R is independently Cl, F, Br, I or H, and z is 1 to 3. A preferred compound of Formula I is tetrafluoropropene, particularly 1,1,1,3-tetrafluoropropene and/or 1,1,1,3-tetrafluoropropene.