Reactively DC-sputtered tungsten trioxide was investigated with respect to the composition W:O:H, W-valencies and electrical resistance in the as-deposited state for possible application as electro- or gasochromic material. The extent of colouration levels obtained with a dispersed Pt-coverage in a H-containing atmosphere were correlated with the analytical results. Possible explanations for the participation of different tungsten valencies in the charge transfer process underlying the optical absorption in the coloured state are presented.