The reaction of [Fe-III(TMPyP)(H2O)(2)](5+), TMPyP = meso-tetra-N-methyl-pyridyl-porphine, with NO to form [Fe-II(TMPyP)(NO+)(H2O)](5+) was studied at pH = 1. Stopped-flow techniques were employed to study the reversible binding reaction of NO as a function of concentration, temperature and pressure. The reverse release of NO was studied directly by using [Fe(H2O)(6)](2+) as a scavenger for NO as a function of temperature and pressure. The reported activation parameters, especially the positive activation entropy and volume values, favour the operation of a dissociative interchange (I-d) mechanism. A volume profile for the overall reaction is reported, and the results are discussed in terms of the influence of the positively charged substituents on the porphyrin chelate. The reaction of the product, [Fe-II(TMPyP)(NO+)(H2O)(5+), with NO2- was studied at pH = 4 under pseudo-first-order conditions. Concentration, temperature and pressure dependencies of the reaction were investigated. Possible mechanisms for the nitrite catalyzed reductive nitrosylation reaction are presented. (C) 2004 Elsevier B.V. All rights reserved.