A direct experimental evidence of electron accumulation in the grain boundary of yttria-doped (2.44 and 6.29 mol %) zirconia ceramics with the grain size of about 30 nm is demonstrated. Upon introducing extra electrons into the 2.44 mol % yttria-doped sample by reduction, its specific grain boundary resistivity decreased significantly compared with that of the oxidized sample while the bulk conductivity remained nearly unchanged. This is attributed to the fact that the introduced electrons almost exclusively segregate into the space charge zones. Such an effect was found to be less pronounced in the reduced 6.29 mol % yttria-doped zirconia. (c) 2006 The Electrochemical Society.