C-O bond activation of DPEphos occurs upon mild heating in the presence of [Ru(NHC) 2 (PPh 3 ) 2 H 2 ] (NHC = N-heterocyclic carbene) to form phosphinophenolate products. When NHC = IEt 2 Me 2 C-O activation is accompanied by C-N activation of an NHC ligand to yield a coordinated N-phosphino-functionalised carbene. DFT calculations define a nucleophilic mechanism in which a hydride ligand attacks the aryl carbon of the DPEphos C-O bond. This is promoted by the strongly donating NHC ligands which render a trans dihydride intermediate featuring highly nucleophilic hydride ligands accessible. C-O bond activation also occurs upon heating cis -[Ru(DPEphos) 2 H 2] . DFT calculations suggest this reaction is promoted by the steric encumbrance associated with two bulky DPEphos ligands. Our observations that facile degradation of the DPEphos ligand via C-O bond activation is possible under relatively mild reaction conditions has potential ramifications for the use of this ligand in high temperature catalysis.