The addition of probe molecules, such as ethylene, propylene, methanol, ethanol, and acetaldehyde, to COH2 feed was studied over the KCl-promoted and the unpromoted Mo SiO2 catalyst under synthetic conditions, in order to clarify the reaction paths for the formation of hydrocarbons and alcohols. The results of olefin addition study suggest that the alcohol formation from COH2 proceeds by a mechanism including steps identical with those in the hydrocarbonylation of olefins and that KCl suppresses the simple hydrogenation of olefins. Both of the Mo catalysts demonstrate a poor catalytic activity for the homologation of methanol to ethanol. Alcohol homologation seems to be only a minor process in the formation of C+2 alcohols. No significant activity of Mo catalysts for incorporation of acetaldehyde into C3 oxygenated compounds may exclude the intermediacy of aldehydes for the chain growth of alcohols. Hydrogenation of acetaldehyde to ethanol is the fast and predominant reaction, in agreement with the fact that alcohols compose more than 90% of the organic oxygenates produced from COH2. Aldol condensation is apparently unimportant for the chain growth. The contribution of alcohol dehydration to hydrocarbon formation may be insignificant over the KCl-promoted catalyst with K Mo = 0.4. Consequently, a mechanism including CO insertion into the alkyl-metal bond is proposed for the main reaction path for the higher alcohol formation from COH2. The role of K is supposed to slow the hydrogenation of surface alkyl species to form alkanes as well as to increase the active sites for alcohol formation by retarding the reduction of Mo.