The reaction of the Pt(I)Pt(I)Pt(II) triangulo cluster Pt-3(mu -PBu2t)(3)(H)(CO)(2) (1) with TfOH (Tf = CF3SO2) affords the hydride-bridged cationic derivative [Pt-3(mu -PBu2t)(2)(mu -H)((PBu2H)-H-t)(CO)(2)]OTf (2). With TfOD the reaction gives selectively [Pt-3(mu -PBu2t)(2)(mu -D)((PBu2H)-H-t)(CO)(2)] OTf (2-D-1), implying that the proton is transferred to a metal center while a P-H bond is formed by the reductive coupling of one of the bridging phosphides and the terminal hydride ligand of the reagent. The reaction proceeds through the formation of a thermally unstable kinetic intermediate which was characterized at low temperatures, and was suggested to be the GO-hydrogen-bonded (or protonated) [Pt-3(mu -PBu2t)(3)(H)(CO)(2)]. HOTf (3). An ab initio theoretical study predicts a hydrogen-bonded complex or a proton-transfer tight ion pair as a possible candidate for the structure of the kinetic intermediate.