Depending on the oxidation state of the TiO2(001) surface, acetaldehyde can form C-4 products by either aldol condensation or reductive coupling routes. Aldol condensation generates crotonaldehyde and crotyl alcohol, and is favored on oxidized, stoichiometric surfaces. Reductive coupling produces butene, and occurs only on reduced surfaces. The switch-over in selectivity with increasing extents of surface oxidation provides important insights into the active site requirements for each of these reactions.