To provide additional evidence on the synthesis path of methanol over Cu/ZrO2 catalysts, a three-step in situ experiment was performed in a DRIFT cell. C-13-labelled as well as common C-12 paraformaldehyde and formic acid were adsorbed and subsequently hydrogenated under reaction conditions (0.5 MPa and 423 K). It was found that methanol was generated exclusively from the eta(2)-H2CO intermediate, not from adsorbed surface formate. Furthermore, the intermediacy of CO32- in the reverse/water-gas shift reaction was shown.