Silica impregnated in aqueous solutions of ammonium heptamolybdate to achieve different Mo loadings (3-15 wt.%) is studied by the Fourier transform infrared/photoacoustic (FT-IR/PA) technique. Special attention is given to the high frequency region of surface hydroxyl groups. Air calcination at 500°C of the Mo loaded silica caused the evolution of defined features at 3720 and 3550 cm-1 which are attributed to the free and hydrogen-bonded components of a pair of hydrogen-bonded silanol groups. The presence of such silanols on the surface of silica heated at 500°C is attributed to the hydrolysis of open siloxane bridges by the action of constitutional water that is liberated from the bulk interior due to heating at 500°C. The open siloxane represents a site of weakly attached Mo species that escape from the surface upon heating. These detached species are seen as the precursor of MoO] detected on the surface (band at 994 cm-1). A surface polymolybdate phase is also detected (band at 970 cm-1) and considered to result from condensation between Mo species adsorbed on the surface