Abstract The deuteron and niobium magnetic resonance in the Nb–D system have been measured over a wide range of deuterium concentrations. It has been found that the order-disorder (β→α) phase transition temperature depends on the small amount of interstitial impurities as well as on the concentration of the deuterium. From the concentration dependency of deuterium (Nb) in the β phase, one can determine the amount of deuterium (Nb) in the β phase coexisting with that in the α phase. On the basis of this finding, a tentative Nb–D phase diagram has been proposed. It has been confirmed that the interstitial impurities decrease the deuterium concentration of the β phase which is in equilibrium with the α phase. The niobium signal intensity has been found to be greatly reduced by the interstitial oxygen and other impurities, while the line width is not affected. For the interpretation of these findings, a simple model has been proposed in which only the interstitial atoms, which are present within the spherical sphere of a critical radius, rc, from a niobium atom, eliminate the niobium resonance by means of the second-order quadrupole interaction.