The kinetics of the reaction between acetylperoxy radicals and nitric oxide have been examined both by transient IR absorption and by time-resolved UV spectroscopy. The former technique enables measurements of NO loss and NO2 formation, whereas the latter provides data on the decay of acetylperoxy radicals, the secondary formation of methylperoxy radicals, and their conversion into methylnitrite, The two methods give consistent rate constants which are fit by the expression k(1) = (2.1(-0.8)(+1.4)) x 10(-12)e((570+/-140)/T) cm(3) s(-1). The room temperature value of k(l) = (1.4 +/- 0.2) x 10(-11) cm(3) s(-1) is somewhat smaller than the currently recommended value, which is based on indirect determinations of k(1). Measurements of the CH3O2 and NO2 yields indicate that the reaction proceeds exclusively to the products CH3C(O)O and NO2. The negative temperature dependence suggests that the reaction proceeds via an intermediate adduct that rearranges and dissociates into the products.