Additives on a 1% V2O5/TiO2 catalyst exhibit two types of interactions with the surface vanadium oxide phase which are observed by Raman spectroscopy under dehydrated conditions and methanol oxidation. Under dehydration conditions, noninteracting additives (WO3, Nb2O5, and SiO2) coordinate directly to the oxide support without significantly interacting with the surface vanadium oxide phase. Furthermore, the effect of the noninteracting additives on the surface vanadium oxide phase is independent of the order of preparation or precursor used. These noninteracting additives do not affect the methanol oxidation activity and selectivity. Interacting additives (K2O and P2O5), however, directly coordinate with the surface vanadium oxide phase. Addition of K2O progressively titrates the surface vanadium oxide sites, as observed from the changes in the structure and reactivity of the surface vanadium oxide phase. The effect of P2O5 on the surface vanadium oxide phase depends on the concentration and sequence of preparation. Addition of higher concentrations of P2O5 forms vanadium phosphate compounds, and results in a change in methanol oxidation activity and selectivity. The addition of 1% V2O5 to a 5% P2O5/TiO2 sample, however, does not show any evidence of compound formation, but a part of the surface vanadium oxide phase appears to be titrated. Under ambient conditions, the additives change the pH at point of zero charge of the surface moisture layer which controls the structure of the surface vanadium oxide layer. Thus, depending on the nature of the additive, interacting or noninteracting, the dehydrated structure and reactivity toward methanol oxidation of the surface vanadium oxide phase are affected or remain essentially unchanged, respectively.