Abstract: The invention concerns non-azeotropic fluid mixtures useful in compression thermodynamic cycles, for example in heat pumps, comprising R23, R142b and optionally a fluid (c) other than R23 or R142b, selected from the group formed of methane and ethane halogenated compounds having a boiling point from -85 to +60.degree. C., in such molar proportions that the R23/R142b ratio range from 0.01:1 to 1.10:1 and the (R23+R142b)/(R23+R142b +c) molar ratio from 0.26:1 to 1:1.The mixtures according to the invention can be used as working fluid for heat pumps whose fluid circulates through an evaporator (E.sub.1), a compressor (K.sub.1), a condenser (E.sub.2) and a pressure reducer (D.sub.1). The thermal exchanges take place respectively with external fluids (5,6) and (7,8).The mixtures according to the invention may supply heat to the condenser at a temperature from +50 to +100.degree. C. while having an internal pressure generally limited to 0.3 MPa.