Single-phase samples of (\mathrm{La}{3} \mathrm{Ni}{2} \mathrm{O}{7-\delta}) with (\delta=0,0.08), and 0.65 were prepared and characterized by powder (\mathrm{X})-ray diffraction, TGA, electrical resistivity, and magnetic susceptibility. The crystal symmetry of the as-prepared ((\delta=0.08)) and oxygenated ((\delta=) 0.00) samples is orthorhombic (space group Fmmm). Oxidation of the as-prepared (\mathrm{La}{3} \mathrm{Ni}{2} \mathrm{O}{6.92}) sample to (\mathrm{La}{3} \mathrm{Ni}{2} \mathrm{O}{7.00}) shows little change in the unit-cell parameters. Reduction of (\mathrm{La}{3} \mathrm{Ni}{2} \mathrm{O}{6.92}) to (\mathrm{La}{3} \mathrm{Ni}{2} \mathrm{O}{6.35}) results in a structural phase transition from orthorhombic to tetragonal symmetry, a dramatic decrease in the (c) cell parameter, and significant increase in the (a) and (b) cell parameters. These data are consistent with loss of oxygen from the apical octahedral sites connecting two perovskite layers in (\mathrm{La}{3} \mathrm{Ni}{2} \mathrm{O}{7-\delta}). The electrical transport properties change with decreasing oxygen content from metallic for (\mathrm{La}{3} \mathrm{Ni}{2} \mathrm{O}{7.00}) to semiconducting for (\mathrm{La}{3} \mathrm{Ni}{2} \mathrm{O}{6.92}) and (\mathrm{La}{3} \mathrm{Ni}{2} \mathrm{O}{6.35}). The fully reduced sample, (\mathrm{La}{3} \mathrm{Ni}{2} \mathrm{O}{6.35}), shows a linear dependence of in (\rho) versus (T^{-1 / 4}) in the temperature range (\mathbf{9 0 - 1 4 0} \mathrm{K}), suggesting a variable-range hopping mechanism. A nearly temperature-independent magnetic susceptibility is observed in the temperature range (100-300 \mathrm{~K}) for the (\delta=0) and 0.08 samples. The relatively high values and the temperature dependence of the magnetic susceptibility indicate a Stoner enhanced Pauli paramagnetism for (\boldsymbol{\delta}=\mathbf{0}) and evidence for a weak ferromagnetism in the (\delta=0.65) sample. The electrical and magnetic properties of (\mathrm{La}{3} \mathrm{Ni}{2} \mathrm{O}_{7-\delta}) are discussed in terms of the mixed-valent character of nickel and vacancy-induced localization effects. O 1994 Academic Press, Inc.