Detailed solution kinetic and equilibria data (mainly in toluene) are presented for the reversible binding of CO and O2 to the five-coordinate hemes Fe(por)B, where B is 1,5-dicyclohexylimidazole or 1,2-dimethylimidazole (chosen to mimic the R- and T-states, respectively) and por = the dianion of some durene-capped porphyrins with variable length linking methylene straps on either side of the durene moiety (4/4, 5/5, or 7/7 methylenes). Use of spectrophotometric equilibrium titrations from 30 to -50-degrees-C, stopped-flow data, and laser flash photolysis under either CO or CO/O2 mixtures, has allowed for determination of on and off rates, equilibrium constants, and, in the case of the 4/4-system, thermodynamic constants for the binding. Increasing steric hindrance provided by the durene cap, in the order 7/7 < 5/5 < 4/4, is generally less than expected from studies with other heme derivatives; in combination with the complete absence of polarity effects, as within nonpolar distal sites, the durene hemes exhibit poor differentiation between CO and O2. However, the distorted 4/4-derivative discriminates between CO and O2 in a novel way through a "proximal effect" associated with deformation of the porphyrin skeleton from planarity, the effect being largely reflected by an increased CO dissociation rate.