The termolecular rate constants for the O(P-3) + (NO)-N-14 + Ar --> (NO2)-N-14 + Ar and O(P-3) + (NO)-N-15 + Ar --> (NO2)-N-15 + Ar reactions were determined under room temperature bulk conditions. O(P-3) was produced by the pulsed photodissociation of SO2 at 210.4 nn and was monitored by two-photon laser-induced fluorescence at 225.7 nm. The rate constants for (NO)-N-14 and (NO)-N-15 were determined to be 5.4 +/- 0.2 and 6.1 +/- 0.3 x 10(-32) cm(6) s(-1), respectively. The error limits are twice the standard errors (S.E.). This isotope effect is opposite to that expected from a statistical model but is similar to that observed in the O-3 formation reactions from O(P-3) and O-2.