[en] The inhibition of positronium formation by the electron scavenger NO₃^- has been measured in H₂O and D₂O. The positronium yields, up to 0.5M concentration of NO₃^-, are persistently lower in D₂O than those in H₂O. No significant difference in the efficiency of NO₃^- inhibiting the positronium formation in H₂O and D₂O has been found. The inhibition coefficients are 2.2 +- 0.3 M^-1 in H₂O and 2.5 +- 0.3 M^-1 in D₂O. These results correlate reasonably well with radiation chemistry results and can be interpreted within the framework of the spur model of positronium formation. (author)