Abstract In the present work the hydrogen absorption-desorption behaviour of the Nd 5Co 2B 6 ρ-boride has been studied. A two-step hydrogenabsorption, corresponding to hydrogenation and subsequent disproportionation, was observed forthis compound in the temperature range 20–860°C. The first event is associated with ananisotropic expansion of the rhombohedral lattice of intermetallic compound in the direction (up to 12.4 vol%) and results in the formation of an interstitial Nd 5Co 2B 6H 7.0 type hydride. The second hydrogen absorption event, takingplace at temperatures above 800°C, results in disproportionation into NdH 3-x, NdCO 2B 2, NdB 4 and a previously unreported neodymium diborideNdB 2. A significant refinement of the microstructure occurs as a result of therecombination reaction to form the original Nd 5Co 2B 6 ρ-boride as the final step on theHydrogenation–Disproportionation–Desorption–Recombination (HDDR) process.