A high-pressure NMR cell based upon the use of a sapphire NMR tube and simple valve design was constructed. The use of the cell to study the hydrogenation of imines under 1000-2000 psi of hydrogen mediated by [tertiary bis(phosphine)]rhodium(I) catalysts is described. Complexes of the type [{Rh(diphos)}2(mu-H)(mu-X)] (X = Cl, I, OMe), which are formed at these elevated hydrogen pressures, were characterized in situ, and the possible significance of these complexes in the catalysis is discussed. The dihydride cis-[RhH2(diphos)2]Cl, produced in a side reaction, was spectroscopically characterized at low temperature under 1200 psi of H-2.