Frozen CH4 and CH4/Ar were irradiated by protons and 3 He 2+ ionsat15and77Kinordertosimulatetheinteraction between the solar wind and the surface of comets. After irradi- ation, polymerised organic matter is left over as solid residues, whenwarminguptheremainingCH4.Thecarbonisotopiccom- positionoftheseresidueswasdetermined.Isotopicfractionation between organic residues and methane reveals a two step pro- cess in the maturation of organic matter: 1) a polymerisation of methane associated with an isotopic fractionation of 16 ; 2) a sputtering of the polymerised species associated with an isotopic fractionation lying between 8 and 15 and with a cor- responding conversion yield of the polymers (i.e. the number of carbon atoms converted into refractory polymers relative to those lost by sputtering) between 9 and 5%. From the present data and assuming that these experiments represent cometary surface conditions, it can be concluded that irradiation of comets by the solar wind does not contribute sig- nicantly to the carbon isotopic fractionation between surface ices, surface polymers derived from these ices and their corre- sponding sputtered gaseous organic species lost in space. Ac- cordingly the mean 12 C/ 13 C ratio of the gas phase should be