The reactivity of hydrogen towards the lanthanum silicides, La 5 Si 3 , LaSi and LaSi 2 , has been studied by gravimetric and differential thermoanalytical methods. Synthesized silicides and hydrogenated phases have been characterized by X-ray diffraction. The hydrogen interaction is attributed to the lanthanum content of the silicide phase. Whereas LaSi 2 does not react with hydrogen over a wide pressure–temperature range, La 5 Si 3 takes up hydrogen even at 100°C. Hydrogenation of La 5 Si 3 leads from a metastable hydride with about one hydrogen atom per lanthanum atom to the decomposition into lanthanum hydride, LaH 2+x , and another phase without X-ray pattern. On the contrary, hydrogen release is not complete at temperatures up to 900°C. LaSi reacts reversibly with hydrogen under formation of a ternary hydride phase.