Comparative studies of axial CO and solvent coordination to iron(II)) protoporphyrin IX ((FePPIX)-P-II) anchored to the surface of mesoporous nanocrystalline (anatase) TiO2 thin films ((FePPIX)-P-II/TiO2) immersed in dimethyl sulfoxide (DMSO), pyridine (py), and methanol (MeOH) and to (FePPIX)-P-II in fluid DMSO and py solution are reported. The equilibrium constants, K-eqCO(S), for CO coordination to (FePPIX)-P-II/TiO2 immersed in py (2.4 x 10(3) M-1) < DMSO (6 x 10(4) M-1) < MeOH (2.3 x 10(5) M-1) were quantified. The corresponding values in fluid py or DMSO solution were similar to 2 times larger (4.5 x 10(3) and 1 x 10(5) M-1, respectively). The observed ligand exchange rates (k(obs)) measured after pulsed 532 nm laser excitation (5-6 ns fwhm, 1-3 mJ/pulse) of (S)(CO)(FePPIX)-P-II/TiO2, where S is solvent, in saturated CO solutions were measured: py (2.2 s(-1)), DMSO (460 s(-1)), MeOH (2.09 x 10(5) s(-1)). The corresponding values in fluid solution were 2.0 s(-1) (py) and 230 s(-1) (DMSO). The observed ligand exchange rate varied linearly with [CO], and second-order rate constants were determined for (FePPIX)-P-II/TiO2 immersed in DMSO (3.1 x 10(5) M-1 s(-1)) and MeOH (1.5 x 10(7) M-1 s(-1)). The observed rate for CO addition to (py)(2)(FePPIX)-P-II/TiO2 immersed in py did not vary linearly with [CO]. The relevance of the measured kinetics and thermodynamics to a dissociative mechanism for ligand exchange is discussed.