In the JET MKIIA divertor a strong asymmetry of carbon deposition between inner and outer divertor leg is observed with deposition at the inner leg - particular on the inner louvres - and essential no or very little deposition in the outer divertor including the louvre region. Thick carbon deposits on the inner louvres contain the majority of the hydrogenic species retained on a long term basis. Understanding and modelling the carbon deposition is of particular importance to estimate the long term tritium inventory and the lifetime of target plates in future fusion devices. This contribution presents Monte-Carlo simulations of the local transport of chemically eroded methane molecules in the divertor of MKIIa using an adapted version of the ERO-TEXTOR code.