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Hydrogenation reactions of CO in inter- and circumstellar ices are regardedas an important starting point in the formation of more complex species.Previous laboratory measurements by two groups on the hydrogenation of CO icesresulted in controversial results on the formation rate of methanol. Our aim isto resolve this controversy by an independent investigation of the reactionscheme for a range of H-atom fluxes and different ice temperatures andthicknesses. Reaction rates are determined by using a state-of-the-art ultrahigh vacuum experimental setup to bombard an interstellar CO ice analog withroom temperature H atoms. The reaction of CO + H into H2CO and subsequentlyCH3OH is monitored by a Fourier transform infrared spectrometer in a reflectionabsorption mode. In addition, after each completed measurement a temperatureprogrammed desorption experiment is performed to identify the produced species.Different H-atom fluxes, morphologies, and ice thicknesses are tested. Theformation of both formaldehyde and methanol via CO hydrogenation is confirmedat low temperature (12-20 K). We confirm, as proposed by Hidaka et al., thatthe discrepancy between the two Japanese studies is mainly due to a differencein the applied hydrogen atom flux. The production rate of formaldehyde is foundto decrease and the penetration column to increase with temperature. In orderto fully understand the laboratory data, the experimental results areinterpreted using Monte Carlo simulations. This technique takes into accountthe layered structure of CO ice. Temperature-dependent reaction barriers anddiffusion rates are inferred using this model. The model is extended tointerstellar conditions to compare with observational H2CO/CH3OH data.

Hydrogenation reactions in interstellar CO ice analogues