Despite early claims that nitric oxide does not react with cobalamin under any circumstances, it is now accepted that NO has a high affinity for cobalamin in the 2+ oxidation state [Cbl(II)]. However, it is still the consensus that NO does not react with Cbl(III). We confirmed that NO coordinates to Cbl(II) at all pH values and that Cbl(III) does not react with NO at neutral pH. At low pH, however, Cbl(III) does react with NO by way of a two-step process that also reduces Cbl(III) to Cbl(II). To account for the pH dependence, and because of its intrinsic interest, we also studied reactions of NO with cobinamide [Cbi] in the 2+ and 3+ oxidation states. Both Cbi(II) and Cbi(III) react readily with NO at all pH values. Again, Cbi(III) is reduced during the process of coordinating NO. Compared to cobalamin, cobinamide lacks the tethered 5,6-dimethylbenzamidazolyl moiety bound to the cobalt ion. It may, therefore, be considered a "base-off" form of Cbl. To explain the reaction of Cbl(III) at low pH, we infer that the base-off form of Cbl(III) exists in trace amounts that are rapidly reduced to Cbl(II), which then binds NO efficiently. Base dissociation, we postulate, is the rate-limiting step. Interestingly, Cbi(II) has 100 times greater affinity for NO than does Cbl(II), proving that there is a strong trans effect due to the tethered base in nitrosyl derivatives of both Cbl(II) and Cbl(III). The affinity of Cbi(II) for NO is so high that it is a very efficient NO trap and, consequently, may have important biomedical uses.