The reactions of nitric oxide and carbon monoxide with water soluble iron and cobalt porphyrin complexes were investigated over the temperature range 298-318 K and the hydrostatic pressure range 0.1-250 MPa [porphyrin ligands: TPPS = tetra-meso-(4-sulfonatophenyl)porphinate and TMPS = tetrameso-(sulfonatomesityl)porphinate]. Large and positive DeltaS(double dagger) and DeltaV(double dagger) values were observed for NO binding to and release from iron(III) complexes Fe-II(TPPS) and Fe-III(TMPS) consistent with a dissociative ligand exchange mechanism where the lability of coordinated water dominates the reactivity with NO. Small positive values for DeltaS(on)(double dagger) and DeltaV(on)(double dagger) for the fast reactions of NO with the iron(II) and cobalt(II) analogues (k(on) = 1.5 x 10(9) and 1.9 x 10(9) M-1 s(-1) for Fe-II(TPPS) and Co-II(TPPS). respectively) indicate a mechanism dominated by diffusion processes in these cases. However, reaction of CO with the Fell complexes (k(on) = 3.6 x 10(7) M-1 s(-1) for Fe-II(TPPS)) displays negative DeltaS(on)(double dagger) and DeltaV(on)(double dagger) values, consistent with a mechanism dominated by activation rather than diffusion terms. Measurements of NO dissociation rates from Fe-II(TPPS)(NO) and Co-II(TPPS)(NO) by trapping free NO gave k(off) values of 6.3 x 10(-4) s(-1) and 1.5 x 10(-4) s(-1). The respective M-II(TPPS)(NO) formation constants calculated from k(on)/k(off) ratios were 2.4 x 10(12) and 1.3 x 10(13) M-1, many orders of magnitude larger than that (1.1 x 10(3) M-1) for the reaction of Fe-III(TPPS) with NO.