The crystal structure, oxidation state, electronic properties of unusual niobates which contain niobium clusters condensed low dimensionally, were investigated by (\mathrm{X})-ray diffraction, (\mathrm{X})-ray photoelectron spectroscopy (XPS), electrical resistivity measurements, and Raman spectroscopy. The results of XPS experiment indicate that there are three niobium oxidation states (\left(\mathrm{Nb}^{4+}, \mathrm{Nb}^{3+}, \mathrm{Nb}^{2+}\right)) in (\mathrm{Ba}{2} \mathrm{Nb}{5} \mathrm{O}{9}) and these oxidation states are expected to be stable considering their atomic coordinates. Pure (\mathrm{BaNbO}{3}) phase was synthesized and its niobium oxidation state was examined in order to compare with those of niobium in (\mathrm{Ba}{2} \mathrm{Nb}{5} \mathrm{O}{9}). Electrical resistivity measurements on (\mathrm{Ba}{2} \mathrm{Nb}{5} \mathrm{O}{9}) show negative (d \rho / d T) behavior between (160 \mathrm{~K}) and (600 \mathrm{~K}). Similar behavior was observed in yttrium substituted niobates, (\mathrm{Ba}{2-x} \mathrm{Y}{x} \mathrm{Nb}{5} \mathrm{O}{9}). The possibility of relationship between low dimensionality and anomalous properties of these niobates is discussed. 1994 Academic Press. Inc.