Abstract Unit cell dimensions and hydriding characteristics of the non-stoichiometric systems ZrMn 2T 0.8, with T = Mn, Fe, Co, Ni and Cu have been determined. Hydrides of these systems show plateau pressures at 25 C which vary in a systematic way with the atomic number of T, ranging from 0.3 (for T = Fe) to 4 atm (for T = Co). All of these plateau pressures are much greater than that of the hydride of stoichiometric ZrMn 2. There is a “Cobalt Effect” in that the plateau pressure of ZrMn 2Co 0.8 hydride is extraordinarily high and, moreover, the lattice dimensions of ZrMn 2Co 0.8 are small compared to ZrMn 2 and the other ternaries. The origin of the Co effect is as yet unclear. It may originate with the particulars of the site occupancies or special metallic bonding effects.