The reaction of the water-soluble porphyrin with in basic solution leads to the stepwise formation of and . The kinetics of the reaction of with was studied as a function of temperature and pressure. The positive value of the activation volume for the formation of is consistent with the operation of a dissociatively activated mechanism and confirms the six-coordinate nature of the monocyano complex. A good agreement between the rate constants at pH 8 and 9 for the formation of the dicyano complex implies the presence of water in the axial position trans to coordinated cyanide in the monocyano complex and eliminates the existence of under the selected reaction conditions. Both and bind nitric oxide (NO) to form the same nitrosyl complex, namely, . Kinetic studies indicate that nitrosylation of follows alimiting dissociative mechanism that issupported by the independence of the observed rate constant on [NO] at an appropriately high excess of NO, and the positive values of both the activation parameters S⧧ and V⧧ found for the reaction under such conditions. The relatively small first-order rate constant for NO binding, namely, (1.54 0.01) x , correlates with the rate constant for release from the complex, namely, (1.3 $\pm$0.2) x at , and supports the proposed nitrosylation mechanism