Low molybdenum silica (1.2 wt.% molybdenum) has been prepared by impregnation (pore filling method) from an aqueous (NH4)6MoTO27 solution (pH ~ 6). The effect of temperature (373- 673 K) was followed by diffuse reflectance IR Fourier transform spectroscopy (DRIFTS). The interaction between molybdenum species, MoO42-, and silica surface (OH pairs) is recognized at 373 K through the appearance of a band at 905 cm- 1 attributed to an Mo-O-Si linkage, that shifted, and decreased in intensity, with the increase in temperature to appear at 915 cm -~ at 673 K. Bands at 555 (vsMo-O-Mo) and 881 cm-~ (vasMo--O-Mo) are correspondingly developed with temperture, and only at 673 K a band due to MoO 3 (998 cm -~) is recognized. These observations are used to propose a firm correlation between the detached molydenum species upon the breaking of the Mo-O-Si bonds and MoO 3 formation. The spectral changes in the stretching frequency region of surface hydroxyls indicated that as the separation between adjacent hydroxyls (involved in interaction with MoO 2-) increases, the strength of the Mo-O-Si link decreases. The presence of C1- or F on the surface of silica was found to decrease the extent of interaction between the molybdenum species and silica through decreasing the probability of finding vicinal hydroxyls. The formation of polymolybdate phase identified on the surface of molybdenum-supported silica is also inhibited by halogenation.