A series of experiments on the surface reactions of hydrogen and deuterium atoms with solid CO, formaldehyde (H2CO), and methanol (CH3OH) has been performed. Successive hydrogenation of CO on surfaces at $\sim $10 K was found to proceed efficiently via tunneling to produce H2CO and CH3OH on dust grains under the typical conditions of molecular clouds. Formation rates are strongly dependent on the surface temperature and composition. The role of surface reactions in the formation of deuterated formaldehyde and methanol was investigated. The deuterium fractionation of methanol observed in molecular clouds was reproduced experimentally via H-D substitution in solid methanol at an accreting atomic D/H ratio of 0.05-0.1. This is the first evidence that grain-surface reactions can be responsible for fractionation. We have determined several effective rate constants for hydrogenation, deuteration, and H-D substitution to construct the surface reaction network for CO, H2CO, CH3OH, deuterated formaldehyde, and deuterated methanol.
