A low-pressure steady-state technique has been used to investigate the rates and mechanisms of the oxidation of isobutane over H₃[PMo₁₂O₄₀], CuH₄[PMo₁₂O₄₀]₂, Cu₂H₂[PMo₁₂O₄₀]₂, Cu₂.₅H[PMo₁₂O₄₀]₂, and Cu₃[PMo₁₂O₄₀]₂. Observed oxidation products over all catalysts are methacrolein, 3-methyl-2-oxetanone, acetic acid, carbon dioxide and water. The most selective catalyst for methacrolein formation at low temperatures (<496°C) is Cu₂.₅H[PMo₁₂O₄₀]₂, where both Cu(II) reduction and acid sites play a role. The least active catalyst at low temperatures is phosphomolybdic acid followed by Cu₃[PMo₁₂O₄₀]₂. This activity is reversed at higher temperatures. The 3-methyl-2-oxetanone is a unique product and is likely to be the precursor to methacrylic acid. Acetic acid is also probably a precursor to complete oxidation. Catalyst deactivation or restructuring is significant only over H₃[PMo₁₂O₄₀].