The triple-point transition temperature in niobium-hydrogen has been determined from DSC measurements during heating and cooling as a function of hydrogen content for both protium and deuterium. The Delta H values derived for the reaction ¹/₂H(D)₂(g)+ alpha to beta from these DSC measurements and data for the reaction ¹/₂H(D)₂(g)+ alpha to alpha ' are -48.7 kJ/mol H and -52.2 kJ/mol D. Hysteresis for the triple-point transition has been measured and found to be the same for niobium-protium and niobium-deuterium. The magnitude of the temperature hysteresis is related to the expected free energy dissipation for the phase transition which occurs through the triple point. The magnitude of the isotope effect for the triple point is considered theoretically.