The vapor-phase aldol condensation of methyl propionate (MP) with formaldehyde (HCHO) to form methyl methacrylate (MMA) was studied with a HCHO/MP molar ratio of 0.2, using formalin as a source of HCHO over solid base catalysts. Cesium hydroxide was the best base catalyst and silica gel was the sole compound effective as the support. The optimum Cs/Si atomic ratio was in the range of 2.0/100 to 2.5/100. Na cations present in the silica gel as an impurity decreased the yield of MMA. The addition of water into the reaction feed did not affect the selectivity, though the reaction rate decreased a little. With an increase in the HCHO/MP molar ratio, the yield increased, but the selectivity decreased. The catalytic activity decreased very slowly with the time-on-stream. The deactivated catalyst was completely regenerated by calcinations in air at 400 degrees C. (c) 2005 Elsevier B.V. All rights reserved.