Phenol hydroxylase (PH) from Acinetobacter radioresis-tensS13 represents anexampleofmulticomponent aromatic ringmonooxygenase made up of three moieties: a reductase (PHR), an oxygenase (PHO) and a regulative component (PHI). The function of the oxygenase component (PHO), here characterized for the first time, is to bind molecular oxygen and catalyse the mono-hydroxylation of substrates (phenol, and with less efficiency, chloro- andmethyl-phenol and naphthol).