Using atomic beam/surface scattering measurements to investigate the desorption kinetics of low-coverage Pb from Mo(100), we uncover a large entropy difference between Pb atoms at terrace and step sites, which should be general for adsorbates on surfaces at high temperatures. A line shape analysis of the transient desorption signal reveals the presence of two species with different lifetimes on the surface. An Arrhenius analysis of these lifetimes from 1150 to 1320 K provides the prefactors and desorption activation energies (332 and 411 kJ/mol) of these two states. A comparison of these energies to those measured directly via adsorption calorimetry strongly suggests that one state is a terrace-bound species. The other, more strongly bound species is attributed to steps. The more strongly bound step species has the higher rate constant for desorption because of its much larger desorption prefactor (9 x 10(19) vs 5 x 10(15) s(-1)). Within transition state theory, the ratio of these prefactors corresponds to 82 J/(mol K) higher entropy for the terrace species than for the step species. This large entropy difference is quantitatively reproduced by a simple model which assumes the terrace species is a 2D ideal gas parallel to the surface and the step species is a 1,D ideal gas along the step edges. Such a difference will generally exist for adsorbed species when k(B)T exceeds the barrier height for adsorbate diffusion across terraces. A consequence of this large entropy difference is that the defect sites are much less populated relative to terrace sites than would be expected based on enthalpy alone. The measured prefactor for Pb desorption was used to analyze earlier surface lifetime measurements for Pb on MgO(100) to extract adsorption energies for that system, as well.