The use of an advanced in situ methodology that combines in situ Raman spectra during catalytic operation and simultaneous online activity measurement is used to study the performance and structure of supported vanadium oxide catalysts during propane oxidative dehydrogenation (ODH). In situ spectroscopy during reaction conditions, with online activity measurement, comprises the simultaneous use of both catalytic and spectroscopic measurement on the same sample. Thus, the possibility of assessing a structure-activity relationship at a molecular level becomes reliable since the structures observed correspond to the working catalyst, as determined online. It is shown that the structure of supported vanadium oxide catalyst under propane ODH is close to that of the system under dehydrated conditions but may show a very moderate extent of reduction. The extent of reduction is strongly dependent on the O-2/C3H8 ratio in the reaction feed. Raman spectroscopy during reaction conditions with online activity measurement shows that surface polymeric vanadium oxide species are more reducible than isolated surface polymeric species. The reduction of surface polymeric species decreases moderately the conversion values but has no appreciable effect on the conversion and selectivity values during propane ODH reaction. Therefore, the active site for propane ODH reaction on alumina-supported vanadia must be a single VO4 site, and no special arrangement of V sites appears to be critical. It is also discussed that the V-O-V bond may not be critical for this reaction.