Iron can be bound to phenylalanine hydroxylase (PAH) in two environments. The assignment of the electron paramagnetic resonance spectrum of PAH to two, overlapping high-spin ferric signals is confirmed by computer simulation. Both environments are shown to be populated in the crude enzyme. Reconstitution of the apoenzyme demonstrated that the two iron environments are not interconvertible. Oxygen consumption during PAH reduction by tetrahydropterin in the absence of phenylalanine but not in its presence explains the different reduction stoichiometries (tetrahydropterin:enzyme) that have been observed.