The dispersion of hydrogen in PdH_x (0<or=x<or=1) is investigated using a cluster-Bethe lattice approximation. Total energies are computed and are used to calculate the heat of formation of the non-stoichiometric hydride. Good agreement is found with experiment for x>or=0.5 while the disagreement at lower values of x suggests that the basic assumption of the model-that hydrogen is dispersed at random throughout the lattice-is incorrect, and that instead regions of high hydrogen concentration may occur together with regions of almost pure palladium.