The catalytic process of cyclohexanone oxidation by hydrogen peroxide was investigated using in situ NMR spectroscopy in real working conditions. The behavior of the Keggin heteropolyacid H3PMo12O40, used as a model catalyst, was explored before and after adding the oxidant agent. This study revealed the evolution pathways to different reduced states of H3PMo12O40 and its reversible transformation into peroxomolybdate complexes. These latter were identified as the active species for the adipic acid formation, while the acid function of the catalyst was found important for the substrate activation via ketonic-enolic tautomerism. The oxidative mechanism of the cyclohexanone was described through three successive steps to produce adipic acid.