A new interpretation of the temperature programmed desorption (TPD) spectra of hydrogen on a Pd(110) single crystal surface is presented. The transient mean-field model developed accounts for both adsorption on the surface and diffusion of hydrogen into the lattice of the crystal. A new approach for modeling TPD spectra is developed, in which both the isothermal exposure as well as the temperature ramp of the TPD experiment are modeled in a consistent way. We demonstrate that for systems with substantial diffusion of the adsorbate into the substrate’s lattice it is not sufficient to report exposure values. For such systems, exposure time has a far more pronounced effect than the exposure pressure has on the TPD spectra, because diffusion is a strongly time-dependent process. The modeling and experimental results for the H-2/Pd(110) system are in good agreement and suggest the existence of a distinct subsurface state, in addition to the surface and bulk states of hydrogen. Low frequency factors derived from the model for the processes connecting the subsurface with the surface and bulk state emphasize the restricted nature of the corresponding transition states.