Various V/P mole ratios of vanadium substituted Keggin-type phosphomolybdic acids were synthesized by the hydrothermal method. These materials were characterized using several physico-chemical techniques such as X-ray diffraction, FT-IR, N-2-sorption, Raman spectroscopy, P-31 MAS NMR, SEM and NH3-TPD. FT-IR, Raman spectroscopy and P-31 NMR results confirm the formation of the primary structure of the Keggin ion and its crystalline nature is shown clearly by XRD. NH3-TPD results reveal that the acidity of the materials systematically decreases with increasing vanadium content. The Knoevenagel reaction carried out over vanadium substituted phosphomolybdic acid with various V/P mole ratios indicate that the higher V/P mole ratio exhibits better catalytic performance under solvent free conditions. The catalytic properties correlate with the structural properties and the acidity of the materials. [GRAPHICS]