A study has been carried out of the gadolinium–hydrogen system using 155Gd sources (from 155Eu) incorporated in SmF3 and SmPd3. Sources were held at 4.2 K and absorbers at various temperatures. Using the ‘single-line’ Sm(155Eu)Pd3 source, isomer shifts (in mm s–1) relative to GdF3 are: Gd, –0.71 ± 0.02; GdH2, –0.10 ± 0.02; GdH3, –0.11 ± 0.02 at 4.2 K. The hyperfine magnetic field in GdH2 is –306 ± 30 kG (for the metal it is –382 ± 10 kG). GdH3 Is quadrupole split, Vzz=(5.1 ± 0.5)× 1017 V cm–2. Isomer-shift data are qualitatively in accord with the anionic model of hydride formation. Electron density at the gadolinium nucleus decreases most markedly with initial addition of hydrogen and little, if any, on going from GdH2 to GdH3. The reverse is observed for the hyperfine magnetic field at the gadolinium nucleus. Electric and magnetic effects are discussed in relation to other relevant chemical and physical properties of the system.