Abstract A chemical theory of hemeprotein reactivity is presented. It is based upon recent chemical and spectroscopic studies of metalloporphyrins and hemeproteins and upon the established X-ray structures of the iron (III) complexes of hemoglobin, myoglobin and cytochrome c. The theory, in simplest form, can be formulated as a set of three interrelated statements: (i) the reactivity of hemeproteins is determined primarily by the inherent chemical capacities of iron porphyrins; (ii) the inherent chemical capacities of iron porphyrins are modulated by the protein in two ways; (a) by controlling the axial ligands on iron and their accessibility to substitution and (b) by providing a tertiary matrix (protein conformation) in which the porphyrin periphery is “blocked” or “exposed”; (iii) the nature of biological electron transfer in hemeproteins is a peripheral “outer sphere” process. Two corollaries are: (i) a dissociative electron transfer reaction does not ensue in a high field complex of a low valent transition metal to a high field ligand; (ii) ubiquinone can function as an electron transfer agent at the cellular level. The congruence of the theory with the current state of knowledge is presented and some general predictions of hemeprotein reactivity are made.