In this study, evidence is provided that isolated surface vanadia (VO₄) species on SiO₂ can similarly act as thermal heterogeneous catalyst and as heterogeneous photocatalyst. Structurally identical surface VO₄ species catalyze the selective oxidation of methanol both by thermal activation and UV light‐induced. Selectivity to formaldehyde appears to be unity. For the photocatalytic reaction at room temperature, formaldehyde desorption is rate‐limiting. With larger agglomerates or V₂O₅ nanoparticles, on the contrary, only the thermal reaction is feasible. This is tentatively attributed to the different positions of electronic states (HOMO/LUMO, valence/conduction band) on the electrochemical energy scale due to the quantum size effect. Besides providing new fundamental insight into the mode of action of nanosized photocatalysts, our results demonstrate that tuning of photocatalytic reactivity of supported transition metal oxides by adjustment of the degree of agglomeration is feasible.