The preparation and study of Pt(II) H-2 adducts and Pt(IV) dihydride complexes are described. The species of interest are generated by protonation of hydridoplatinum(II) complexes of the type trans-(PCy3)(2)Pt(H)X [X = SiH3, H, CH3, Ph, Cl, Br, I, CN, CF3SO3] and [trans-(PCy3)(2)Pt(H)L][BAr4f] [L = CO, 4-picoline; BAr4f = B(3,5-C6H3(CF3)(2))(4)]. The proton attacks one of three different sites on these complexes (hydride, platinum, or the trans ligand), depending on which ligand is trans to hydride. These studies reveal several factors affecting the stability and reactivity of Pt(II) sigma adducts, which thus have implications for C-H activation by Pt(II).