Using infrared reflection-absorption spectroscopy in conjunction with the broad-band technique of Fourier-transform interierometry, changes m the absorption spectrum of W(100) have been investigated as a function of H coverage at 100 K from 900 to 3900 cm−1. The changes measured upon initial adsorption indicate the removal of intrinsic-surface states 0.36 eV below the Fermi surface as well as the establishment of the unreconstructed-surface electronic absorption at ∼ 0.15 eV. The appearance of the 0.15 eV mode upon initial adsorption implies that the surface largely switches to an unreconstructed state at low coverages, consistent with H-atom immobility at low temperatures. For higher coverages [> 0.7 monolayer (ML) (2.0 ML  saturation)] the measured reflectivity changes show that free-carrier surface scattering dominates the coverage induced variations. It is seen that infrared measurements of these surface-scattering variations can lead to determination of the effective plasma frequency ohmsp of the metal.