Mo(PMe3)(6) and W(PMe3)(4)(eta(2)-CH2PMe2)H undergo oxidative addition of the O-H bond of RCO2H to yield sequentially M(PMe3)(4)-(eta(2)-O2CR)H and M(PMe3) 3(eta(2)-O2CR)(eta(1)-O2CR)H-2 (M = Mo and R = Ph, But; M = W and R = But). One of the oxygen donors of the bidentate carboxylate ligand may be displaced by H2O to give rare examples of aqua-dihydride complexes, M(PMe3)(3)(eta(1)-O2CR)(2)(OH2)H-2, in which the coordinated water molecule is hydrogen-bonded to both carboxylate ligands.