The nature of the products of the selective oxidation of isobutane into methacrolein and methacrylic acid over a Mo-V-P Keggin-type-structure catalyst was investigated at 593 K. Gas chromatography/mass spectrometry and H-1 and C-13 NMR were complementary analytical methods to identify by-products. The quantification was realised by H-1 NMR and gas chromatography. Aromatic and linear carbon backbone products detected suggested that an isomerization occurred during the oxidation of isobutane. A better knowledge of the nature of the acid site on which isomerization occurs might design a powerful catalyst. (c) 2006 Elsevier B.V. All rights reserved.