A method and an apparatus for the largest recovery of ethylene and propylene from gas produced by the pyrolysis of hydrocarbons, wherein any liquid current produced by the fractional condensation of the raw gas up to a definite temperature is directly fed to an ethane fractionating column (deethanizer) the bottom product of which is the purified current containing almost all propylene, while any current condensed at lower temperature is fed to a methane separating column (demethanator) provided with a reboiler in which the partial condensation of the overhead vapor of the deethanizer occurs, and the bottom product of the demethanator contains all ethylene.