Abstract: Method for controlling defrosting of the outer exchanger of a heat pump machine, comprising the steps of: defining in the outer air temperature/evaporation temperature plane three zones; acquiring the outer air temperature and the evaporation temperature; individuating in which zone of the Taria_ext/T_evap plane the point falls, identified by the two acquired temperature measures; inverting the machine functioning cycle when the point falls in an unsafe zone; reducing the rotation speed of the compressor of said heat pump machine when the point falls in an intermediate zone.