Processing of nanocrystalline Bi2O3-Y2O3 solid electrolyte having high density has been investigated and reported in this paper. Nanopowders of mixed bismuth oxide and yttrium oxide (75 mol% Bi2O3 + 25 mol% Y2O3) have been prepared by reverse titration chemical coprecipitation from Bi3+ and Y3+ containing aqueous solution. The high density, nanocrystalline Bi2O3-Y2O3 solid electrolyte has been synthesized by pressureless reactive sintering. The XRD results of the formation of nano delta-Bi2O3 and its grain growth indicates that the solid solution reaction between Y2O3 and beta-Bi2O3 occurs essentially in the initial stage of sintering and the growth of nano delta-Bi2O3 crystallites obeys the parabolic rate law, expressed as (D - D-0)(2) = K(.)t, during sintering process. After sintering at 600 C for 2 h, the relative density of the samples has been found to be greater than 96%. The microstructure reveals that the nanocrystalline Bi2O3-Y2O3 solid electrolyte samples have dense equiaxed microstructure with few isolated pores and delta-Bi2O3 grains are less than 100 nm in diameter. (c) 2005 Elsevier B.V. All rights reserved.