The vapor-phase aldol condensation of propionic acid (PA) with formaldehyde (HCHO) to form methacrylic acid (MAA) was performed with a PA/HCHO molar ratio of 2 using various silica-supported metal phosphate catalysts. In the absence of water, a silica-supported tin phosphate with Sn/Si/P atomic ratio of 1/8/2.3 showed the same level of performance as a silica-supported vanadium phosphate with V/Si/P atomic ratio of 1/8/2.2 did. The one-pass yield of MAA reached 58% based on the HCHO feed. In the presence of water, the performances of almost all catalysts fell. However, the Sn/Si/P catalyst showed a relatively high performance even in the presence of water. The performance is almost identical to that obtained from a silica-supported cesium oxide catalyst. (C) 2003 Elsevier B.V. All rights reserved.