Experiments of deuterium absorption/desorption were performed on a foil of Pd77Ag23 (wt %) alloy in the 78-196 degrees C temperature and 1-4 bar pressure ranges under the neutron beam at ILL (Grenoble). Powder diffraction patterns were collected on the D1B diffractometer (lambda = 1.2871 angstrom) on the Pd0.772Ag0.228Dv deuterated alloy, and the face-centred-cubic structure was Rietveld-refined locating the D atom in the octahedral site with a variable occupancy v (=D/M ratio). The results of v(T) curves at different p(D-2) pressures are discussed in comparison with literature data on H and D absorption into Pd-Ag alloys from thermodynamic measurements. A negative deviation from predictions of Sieverts' law is shown by the dependence of D occupancy on pressure. This effect relates the behaviour of the activity coefficient TD to D-D and D-M interactions in the solid solution phase. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.