Microgravimetric measurements are reported of the pressure-concentration dependence of deuterium in the palladium solid-solution alloy, Pd 0 . 9 Y 0 . 1 . An Intelligent Gravimetric Analyzer (IGA) was used and the measurements were made in the temperature range (60°C≤T≤100°C). The results obtained revealed a hitherto unseen hysteresis effect in the absorption-desorption isotherm. This hysteresis is believed to be due to a reordering of the palladium-yttrium lattice driven by the hydrogen (deuterium) absorption. The second absorption/desorption cycle shows a wider hysteresis loop than the first but further cycles repeat the shape of the second loop. In-situ neutron diffraction measurements were also made on deuterated Pd 0 . 9 Y 0 . 1 at 303 K. These data support the hypothesis that the hysteresis is due to an order-disorder transition involving the yttrium atoms in the metal lattice.