The fuelling behaviours of CH4 and CO were investigated on the tokamak experiment ASDEX by means of sinusoidally modulated gas puffing. Depending on the gases used quite different penetration behaviours of the carbon impurity were observed. These are attributed to the different fragmentation processes of the CO and CH4 molecules and the corresponding losses of neutral radicals and atoms to the vessel walls, which are higher for CH4. From the greater penetration probability of carbon originating from CO fragmentation, a more serious carbon impurity fuelling by oxygen induced chemical erosion is inferred as compared with that by hydrogen induced chemical erosion