The photosubstitution reactions of M(CO)(4)(phen) (M = Cr, Mo, W; phen = 1,10-phenanthroline) with PR3 (R = Me, Bu-n, Ph) to form M(CO)(3)(PR3)(phen) were studied as a. function of excitation wavelength, entering nucleophile concentration, and pressure. Ligand field photolysis in general results in a dissociative substitution mechanism, whereas charge-transfer photolysis can, depending on the nature of M and PR3, proceed according to an associative mechanism. The chemical and physical variables studied result in a systematic tuning of the photosubstitution mechanism. Nucleophile concentration, excitation wavelength, and pressure dependencies reveal unique mechanistic information. The results are discussed in reference to available literature data, and a complete mechanistic analysis is presented.