feti is an alloy capable of reversible hydrogen storage at reasonable temperatures and pressures: however, it requires an initial activation procedure and experiences losses in sorption capacity after a number of charge-discharge cycles. backscatter conversion electron mossbauer spectroscopy (cems), x-ray photoelectron spectroscopy (xps) and secondary ion mass spectrometry (sims) were used to probe the entire 10 nm, the top 2 nm and the top monolayer respectively of feti thin films after fabrication and various annealing, reduction and oxidation treatments. data from these analyses support an initial ultrahigh-vacuum activation mechanism whereby iron oxides present on the surface are reduced by metallic titanium species in underlying feti layers to produce tio/sub 2/ and metallic iron domains. metallic iron domains are exposed to the gas phase and are capable of dissociating molecular hydrogen. the mechanism responsible for the gradual loss in sorption capacity (i.e. cyclic deactivation) occurs when feti decomposes into metallic iron and tio/sub 2/ by reaction of titanium with p.p.m levels of o/sub 2//h/sub 2/o impurities in the h/sub 2/ charging gas. both initial activation and cyclic deactivation involve decomposition of feti in the surface region