The redox behaviour and the oxygen chemisorption of SiO2, MoO3/SiO2, V2O5/SiO2, TiO2, and V2O5/TiO2 systems have been systematically evaluated by H-2 and CH4 temperature programmed reduction (i.e., H-2-TPR, CH4-TPR) and high temperature oxygen chemisorption (HTOC) measurements, respectively. The influence of the oxide loading on the surface structure and dispersity of MoO3/SiO2 and V2O5/SiO2 catalysts has been assessed. CH4-TPR measurements indicate that the activity-selectivity pattern of oxide catalysts in MPO (A. Parmaliana and F. Arena, 1997, J. Catal. 166, 000-000) is controlled by their capability to interact with CH4 undergoing a redox cycle under reaction conditions. The opposite effect of MoO3 and V2O5 promoters on the catalytic functionality of the "precipitated" silica support has been explained in terms of a different reducibility of MoO3 and V2O5 "surface species." The nature of the support as well as the level of the oxide loading affect the "reactivity" of lattice oxygen of supported systems determining their catalytic behaviour in MPO. The most effective reaction mechanism leading to the primary formation of HCHO implies the direct participation of gas-phase oxygen, while the involvement of bulk-lattice oxygen ions in the process of product formation mainly leads to COx. The interaction occurring between catalyst surface and reaction mixture under steady-state conditions is addressed.