The kinetics of the addition reaction of the ethyl radical and the tert-butyl radical to NO has been investigated at 1.5 bar as a function of temperature (219-475 K) using the muon spin relaxation technique in longitudinal magnetic fields. The data are represented by k(1) = (4.5(-2.1)(+4.1)) x 10(-10) T-0.5+/-0.1 cm(3) molecule(-1) s(-1) for the ethyl radical and k(2) = (5.2(-3.1)(+8.3)) x 10(-4) T-3.1+/-0.2 cm(3) molecule(-1) s(-1) for the tert-butyl radical, or alternatively by Arrhenius parameters of E-a = -(1.4 +/- 0.3) kJ mol(-1) and A = (1.3 +/-0.2) x 10(-11) cm(3) molecule(-1) s(-1) for the ethyl radical and of E-a = -(7.3 +/- 0.4) kJ mol(-1) and A = (5.5 +/- 0.9) x 10(-13) cm(3) molecule(-1) s(-1) for the tert-butyl radical. They provide a basis for tests of absolute rate theories for reactions occurring on more than one potential energy surface.