Mossbauer and EPR studies of catechol 1,2-dioxygenase from Pseudomonas putida and several of its complexes show that the iron center is high-spin ferric in character in all these complexes, including the enzyme-catechol complex and the steady-state intermediate obtained from the reaction of the enzyme, pyrogallol, and O2. The Mossbauer spectrum of the native enzyme consists of two major components whose relative amounts vary with pH and buffer. One component exhibits a spectrum virtually identical with those of protocatechuate 3,4-dioxygenases from Pseudomonas aeruginosa and Brevibacterium fuscum with a common magnetic hyperfine constant, A/gnβn of -21.0 T. This A value appears to be characteristic of this class of dioxygenases. It is closely matched by that of Fe(salen)benzoate (-20.9 T), a synthetic complex that approximates the coordination environment of the metal site. The binding of substrate and inhibitors alters the observed magnetic hyperfine parameters and zero-field splittings. Algnβnchanges for -20.8 T for the phenol complex to -20.0 T for the thiophenol complex, indicating the greater covalency of the Fe-S bond. More interestingly, A/gnβn for the catechol complex, -18.9 T, is the smallest in magnitude observed for a dioxygenase complex and indicates a greater delocalization of unpaired spin density away from the ferric center than in the phenol and thiophenol complexes. The unpaired spin density is presumably transferred onto the catechol, and this may enhance the reactivity of the substrate with dioxygen.