The speciation of the deposited MOx phase (M = Re-VII, Mo-VI, W-VI) in titania-supported transition metal oxide catalysts is studied by in situ Raman spectroscopy under dehydrated conditions at temperatures up to 430 degrees C. An astonishing temperature-dependent heterogeneity is established for MOx phases at coverages roughly less than half the monolayer. Unlimited cycling between the majority high temperature mono-oxo termination configuration (Ti-O)(n)M = O and the di-oxo termination configuration (Ti-O)(n-1)M(= O)(2) that predominates at intermediate temperatures (e.g. 100-150 degrees C) can be achieved by appropriate temperature control and adequate exposure. Support surface hydroxyls and retained water molecules mediate the structural transformation following a proposed molecular level mechanism. Monolayer catalysts (e.g. 5.9 Mo/nm(2)) do not exhibit the reported effect due to the complete titration of support surface hydroxyls during the deposition stage. The inherent extent of hydroxylation of the support affects the extent of the reversible transformation. As a paradigm, a higher degree of reversible transformation is observed for TiO2(P25)-supported catalysts compared to TiO2(anatase)-supported counterparts due to the well-established higher hydroxylation of the former TiO2(P25) support material. TGA analysis and Raman measurements under static equilibrium (p(O2) = 0.23atm) in evacuated and sealed cells extending also in the hydroxyl stretching region further complement the in situ Raman experiments.