Dependent on the application or characterization method catalysts are exposed to different gas pressures, which results in different structures. The quantitative determination of the structure and composition of a catalyst as a function of its gas environment allows the establishment of structure-performance relationships. Herein, we determine the structure of a platinum-titania catalyst under hydrogen during temperature-programmed reduction over 3 orders of magnitude in pressure, from 1 to 950 mbar. The pressure significantly influences the hydrogen uptake kinetics and the consecutive structural transformations of the platinum-titania catalyst. The reduction of the platinum precursor becomes pressure-independent above 30 mbar. Yet, the related spillover and stability of adsorbed hydrogen on the titania are a function of pressure. Higher pressures promote higher hydrogen uptake and prevent desorption of hydrogen from the catalyst. The hydrogen uptake triggers a phase transformation of anatase to rutile which is, as a result, pressure dependent. The presented systematic approach establishes a pressure-structure relation which can be applied for the catalyst treatment and to frame existing results on the catalytic system. Treating the same material at two different pressures will lead to different structures.