A number of Fe(II) porphyrins, previously shown to model oxygen carriers when incorporating a nitrogenous base as a proximal ligand, have been investigated for their ability to model the active site of cytochromes P-450 by using a thiolate group as a fifth axial ligand. In spite of difficulties in controlling the chemical preparation of the thiolate pentacoordinated Fe(lI) porphyrins, association and dissociation rate parameters have been determined for CO and, for the first time, for O-2 binding, using laser flash photolysis. A large decrease of the affinity ratio M = K-CO/K-O2 is observed when the nitrogenous axial ligand is replaced by a thiolate, as observed between hemoglobins and most cytchromes P-450. The changes in the rate parameters suggested including the ethanolate ligand in this study in order to examine more thoroughly the trans influence exerted by the basicity of the proximal ligand upon O-2 and CO binding over a wide range of pK(a) values. The data indicate that the increase of the pK(a) of the proximal Ligand is accompanied by a decrease of O-2 and CO association rates while dissociation rates increase for CO and decrease for O-2. The opposite trend exhibited by the gaseous ligands is discussed in terms of different relative contributions of sigma-bonding and pi-back-bonding to the stabilization of the iron(II)-ligand complexes.