The infrared spectra of CO and CO2 adsorbed on zinc oxide have been observed in the region 2–9 µ. The results show that CO does not exist as such on the surface at 155 °C, but is immediately oxidized to CO2. There is no evidence for carbonate formation in the adsorption of CO, the spectra corresponding to structures of the carboxylate type. A band is observed at 2174 cm−1 for CO at 25 °C which may correspond to dipole interaction or to an oxidation step.