The catalytic role played by praseodymium molybdate phases (Pr6MoO12, Pr2MoO6, Pr2Mo3O12) generated in Pr6O11-MoO3 catalysts during their use in the selective oxidation of isobutene to methacrolein is evaluated in order to determine whether the observed synergetic effects have to be assigned to cooperation between binary praseodymium and molybdenum oxides, or to the occurrence of praseodymium molybdate phases, that would be catalytically active themselves or would act synergetically with MoO3 or Pr6O11 X-ray diffraction, infrared spectroscopy, X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry (ToF SIMS) are used to determine the composition of the used Pr6O11-MoO3 catalysts. Pr6MoO12 appears to be the major ternary phase present in the bulk and Pr2Mo3O12 is predominant at the surface. To understand the catalytic behaviour of the Pr6O11-MoO3 mixtures, the catalytic performances of the pure praseodymium molybdate phases and those of their mixtures with MoO3, Sb2O4 and Pr6O11 were also examined. It is shown that the cooperative effects previously observed in the MoO3-Pr6O11 mixtures most probably cannot be assigned exclusively to the intrinsic catalytic properties of praseodymium molybdates. A cooperation is also observed between other couples of oxide phases, i.e. Pr6MoO12 displays donor properties with respect to MoO3. It appears also that the evolution of the catalytic properties of Pr6MoO12-MoO3 mixtures with respect to the Pr/Mo content is completely different from the one observed in MoO3-Pr6O11 mixtures. In addition, it does not seem that the observed synergetic effects could be assigned to the simultaneous presence of ternary PrxMoyOz phases and Pr6O11. The generation of praseodymium molybdate phases during the catalytic tests seems therefore to play no significant role in the catalytic behaviour of MoO3-Pr6O11 mixtures.