Vapor-phase aldol condensation of butanal to form 2-ethyl-2-hexenal was carried out over several oxide catalysts such as SiO2-Al2O3, Al2O3, ZrO2, and SiO2. Catalysts with moderate and strong acid sites such as Al2O3 and SiO2-Al2O3 were active for the reaction in the initial period, whereas they deactivated rapidly. In contrast, SiO2 with weak acidity showed a low but a stable catalytic activity for the formation of 2-ethyl-2-hexenal. Thermogravimetric analyses of the samples used after the reactions indicate that SiO2 has the smallest amount of carbonaceous species that contributed to its stable activity among the tested catalysts. SiO2 catalysts with different pore sizes and specific surface areas were examined: SiO2 with a mean pore diameter of 10 nm and a surface area of 295 m(2) g(-1) showed the best catalytic performance and gave a 2-ethyl-2-hexenal selectivity of 90% at a conversion of 48% at 240 degrees C. In the catalytic test using deuterated SiO2, which was prepared by contacting SiO2 with deuterated water before the reaction, it was confirmed by a mass spectrometer that the deuterium atom of SiOD was transferred to a 2-ethyl-2-hexenal molecule during the reaction. It is indicated that silanol groups on the SiO2 surface played a role as an active site.