The lattice expansion of Pd during the absorption of hydrogen has been investigated with a combined technique in which the length changes and hydrogen concentration were measured simultaneously and on the same sample in the single-phase region of the PdH c phase diagram above the critical point. For atomic ratios up to c= 0.65, the hydrogen induced relative volume changes are found to be Delta V/V 0= 0.200 c-0.0493 c 2. This concentration dependence of the lattice expansion, as well as that of the partial molar enthalpy Delta H H of hydrogen in Pd, can be described successfully by taking into account the volume and concentration dependences of the characteristic band-structure energy (E F-E s) introduced in the semi-empirical heat of formation model of Griessen and Driessen. Using this model the thermally broadened density of states in PdH c, as a function of band fillings above the Fermi level in pure Pd, is deduced from the experimental data and it is shown that the strong concentration dependence of Delta V/V 0 in PdH c originates from the difference between the volume dependences of the almost completely filled d band at low hydrogen concentrations and the occupied sp band at higher hydrogen concentrations. In NbH c where such ad band to sp band'transition'does not occur the concentration dependences of Delta H H and Delta V/V 0 are dominated by the volume dependence of the partially filled d band.