Hydrogenation of carbon monoxide on the Ru(001) surface has been investigated using high-resolution electron energy loss spectroscopy and temperature-programmed desorption. Exposing gas-phase atomic hydrogen to a saturated carbon monoxide overlayer at 100 K results in reaction (via Eley-Rideal kinetics) under ultrahigh vacuum conditions. Both eta(1)- and eta(2)-formyl are clearly identified as initial reaction products at low atomic exposures. At higher exposures the production of eta(2)-formaldehyde is observed. Annealing to 180 K decomposes some of the eta(1)-formyl, leading to adsorbed CO and hydrogen desorption, with the remainder of the eta(1)-formyl converting to eta(2)-formyl. Upon annealing to 220 K, the eta(2)-formaldehyde decomposes to adsorbed CO and hydrogen which desorbs. Further annealing to 250 K leads to complete decomposition of the eta(2)-formyl, resulting in hydrogen desorption and regeneration of the original CO overlayer. These identifications represent the first spectroscopic observation of a carbonyl insertion channel operating during carbon monoxide hydrogenation on a well-characterized transition metal surface.