Polymerisations at different ethylene pressures and temperatures in the presence of deuterium as a chain transfer agent have been performed with a CrCp2/SiO2 catalyst. The polymers formed have been analysed by H-1, H-2, and C-13 NMR spectroscopy. The results from the NMR analyses, DRIFTS analysis of the catalyst in the presence of ethylene, and theoretical modelling (DFT) have been used to investigate the mechanism for methyl branch formation. Indirect evidence for the presence of an isomerised chromium site has been found, where the chromium is bonded to a secondary carbon atom within the polymer chain. The DFT calculations indicate an energy barrier of 17 kcal/mol for the isomerisation, where a hydrogen atom is transferred from the alpha-carbon to the beta-carbon through an olefin-chromium-hydrid intermediate. Propagation from the isomerised site gives a methyl branch. Ethyl branches are also observed, which may be explained by repeated isomerisation. The energy barrier for propagation from the isomerised site is significantly higher than for the regular site where chromium is bonded to the chain end, with barriers of 12 and 8 kcal/mol, respectively. The results support a model in which the observed strong pressure dependence of the activity on the partial pressure of ethylene is due to a higher rate of isomerisation than propagation from this site at low ethylene pressures.