Temperature-programmed desorption, hydrogenation, and oxidation are used to show that acetaldehyde undergoes continued aldol condensation beyond crotonaldehyde formation on Degussa P-25 titania to form hexan-2,4-diene-al and higher molecular weight compounds containing conjugated C=C bonds. Aromatic compounds also form, and at higher temperatures coke forms. Degussa P-25 TiO2 has more sites that catalyze chain propagation reactions beyond formation of C-4 species than do pure anatase or rutile surfaces. These reactions may be responsible for deactivation observed during photocatalytic oxidation of acetaldehyde at elevated temperatures.