Amorphous films of W03, colored and bleached in electrochemical and in H2-containing gas experiments, were characterized by optical and electrical methods, and the hydrogen content CH was measured by nuclear reaction analysis (NRA) . In the gas phase experiments the kinetics of the coloration process and the optical density show a strong dependence on the H2 partial pressure. The change in CH connected with the electrochromic effect of W03 could be determined by comparing NRA results with optical data. Distinct differences between samples colored electrochemically and by gas phase experiments were observed. The electrochromism of all-solid-state devices was studied also and is compared with the findings on single a-W03 layers.