The selectivity of active sites on oxide catalysts have been assessed by comparing their ability to selectively activate a C-H bond in a reactant rather than a similar C-H or C-C bond in a selective oxidation product. Active sites on oxide catalysts are capable of activating target bonds in reactants that are up to 30-40 kJ mole(-1) weaker than similar bonds in the selective oxidation product. Good selectivities are always recorded provided that selective oxidation reactions attempted do not exceed the discriminating capacity of the active site. Evidence is also presented that C-C bonds, which are generally weaker than C-H bonds, are protected fi-om rupture by steric factors.