The surface infrared reflectance spectra of Mo(100)-p(1 x 1)H and W(100)-p(1 x 1)H are both characterized by two vibrational absorptions in the 800--4000-cm/sup -1/ region: a broad band at 1016 cm/sup -1/ at T = 100 K for H on Mo(100) and at 1069 cm/sup -1/ at T = 300 K for H on W(100), corresponding to the symmetric stretch (..nu../sub 1/), and a narrow derivative-like feature at 1302 cm/sup -1/ on Mo(100) and at 1269 cm/sup -1/ on W(100), identified as the first overtone of the wag motion (2..nu../sub 2/). The physical origin of the line shapes, as well as the larger intensity of the 2..nu../sub 2/ than expected from adiabatic considerations, were investigated through phenomenological line-shape analysis of data obtained as a function of temperature and relative H/D and H/CO coadsorption concentrations. Inhomogeneous broadening is negligible, and dephasing processes contribute weakly to both ..nu../sub 1/ and 2..nu../sub 2/ linewidths. The most distinctive feature of the spectra, the derivative or ''Fano shape'' of 2..nu../sub 2/, arises from a nonadiabatic coupling between the sharp 2..nu../sub 2/ vibration and the continuum absorption due to surface electronic transitions. The asymmetry parameter nu-tilde/sub 0/tau-tilde, which gauges the 2..nu../sub 2/-continuum coupling strength, is temperaturemore » insensitive and does not exhibit an isotopic dependence, as predicted for a strong breakdown of adiabaticity. The narrow linewidths observed for 2..nu../sub 2/ on both surfaces at T = 100 K, 12.2 cm/sup -1/ on Mo(100) and 18.5 cm/sup -1/ on W(100), set a limit on the lifetime of these vibrational levels at T/sub 1/greater than or equal to0.9 ps and T/sub 1/greater than or equal to0.6 ps, respectively.« less