In situ X-ray diffraction (XRD) investigations of the structural changes of Cs1H3PVMo11O40 Keggin type heteropolyacid catalysts show that during the methacrolein (MA) to methacrylic acid (MAA) oxidation reaction the crystallites of the HPA disintegrate to small crystallites of about 10 nm in size. When turning the MA supply repeatedly on and off the observed broadening of the diffraction lines appears and disappears in a rather reversible and reproducible way indicating that the disintegration of the crystallites must be attributed to the catalytic interaction between MA and the HPA catalyst. The reported effect shows some similarities to the formation of the so-called quasiliquid phase by the interaction of H3PW12O40 with polar organic molecules like pyridine or methanol. Presented results suggest that under the conditions of the catalytic reaction MA molecules penetrate into the intracrystalline pore system of the HPA and thus the participation of a quasiliquid phase in the catalytic reaction.