A dimer-of-dimers model compound for the oxygen-evolving complex of photosystem II, [{(H2O)-(terpy)Mn-IV(mu-O)(2)Mn-IV(terpy)}(2)(mu-O)](ClO4)(6) (terpy = 2,2':6',2"-terpyridine), has been prepared and characterized by X-ray crystallography and ESI-MS. Low pH was found to promote the disproportionation of [(Mn2O2)-O-III/IV(terpy)(2)(OH2)(2)](3+) to Mn2+ and a (Mn2O2)-O-IV/IV(terpy)(2) species; the latter complex slowly dimerizes to form the title complex. Protonation of a mu-oxo bridge is proposed to initiate the disproportionation, based on analogy with the [(Mn2O2)-O-III/IV(bpy)(4)](3+) system.