Keggin-type phosphomolybdates with cesium, tellurium and vanadium as counter-cations, recently shown to be efficient for selective oxidation of isobutane, were characterized by diffuse reflectance and Raman spectroscopy. In the present study, it was shown that the Raman bands appearing in addition to those of [PMo12O40](3) in these catalysts after reaction were observed with much better sensitivity using exciting lines at 647.1 and 785 nm. This phenomenon, linked to the existence of d-d transitions and intervalence charge transfers at 680 and 860 nm, respectively, arises mainly from reduction of the probed depth and to a lesser extent, from enhancement effect using resonance Raman spectroscopy. The better sensitivity of this technique was used to follow specifically the additional bands before, during and after reaction. They were attributed to highly reduced phosphomolybdates stabilized under reaction mixture and possibly capped by Te4+ and (V-O)(2+). The reduced species would be present in a layer lying over unreduced Keggin salt and specifically probed using resonance Raman spectroscopy. Substitution by tellurium cations was shown to favour reduction of Keggin heteropolyanions under reaction mixture. Operando resonance Raman experiments evidenced that, for the samples without vanadium, reduced phosphomolybdates were formed above 120 degrees C during the raise up to the reaction temperature whereas for the samples with vanadium, small amount of these species was already observed before reaction because of oxidation of V3+ to V4+ cations during drying. This amount also increased during the thermal activation under reaction feed. The relative intensity of the additional bands was maximal when the catalysts were active and selective under reaction mixture. After reaction, this parameter was related to the selectivity to methacrolein and methacrylic acid suggesting that reduced phosphomolybdates are more selective active sites. (C) 2009 Elsevier B.V. All rights reserved.