The effect of the presence of higher oxygenates on the synthesis of methanol over an unmodified commercial Cu/ZnO/Al₂O₃ catalyst operating under realistic industrial conditions (523 K, 50 barg) was investigated using acetic acid as a suitable probe molecule. On addition of 1 mol% acetic acid the methanol yield decreased by 36%. The acetic acid was totally converted to a range of products namely, ethanol, methyl acetate, carbon monoxide and carbon dioxide. Use of d₄-acetic acid revealed no kinetic isotope effect in the formation of any product. However a range of CX₃CH₂OH species were formed including CD₃CH₂OH, CDH₂CH₂OH and CH₃CH₂OH revealing H/D exchange in the methyl fragment. Using [1-¹³C]-acetic acid and [2-¹³C]-acetic acid it could be shown that the production of CO/CO₂ from the adsorbed acetate was through decarboxylation. The residual adsorbed methyl fragment decomposed to give carbon, which was re-oxidised with adsorbed oxygen.