Phosphorus monoxide (CPO) is a key intermediate in phosphorus chemistry, and its association with the hydroxyl radical (COH) to yield metaphosphorous acid (cis-HOPO) contributes to the chemiluminescence in the combustion of phosphines. When photolyzing cis-HOPO in an Ar-matrix at 2.8 K, the simplest dioxophosphorane HPO 2 and an elusive hydroxyl radical complex (HRC) of CPO form. This prototypical radical-radical complex reforms into cis-HOPO at above 12.0 K by overcoming a barrier of 0.28 AE 0.02 kcal mol À1. The vibrational spectra of this HRC and its D-and 18 Oisotopologues suggest a structure of COH•••OPC, for which a triplet spin multiplicity with a binding energy of À3.20 kcal mol À1 has been computed at the UCCSD(T)-F12a/aug-cc-pVTZ level.