Gas-phase synthesised zirconia powders with yttria additions provide an attractive alternative to conventional co-precipitated powders as a route to zirconia ceramics. Ceramics produced from such powders show improved resistance to hydrothermal ageing [1]. Here we have compared sintering behaviour of a gas phase derived 8 mol% yttria-stabilised zirconia (Tioxide) with a coprecipitated 8 mol% (Tosoh) material. Phase development as a function of sintering/reaction has been studied by X-ray diffraction, ac impedance and electron optical techniques. As sintering temperature was increased the expected transformations from monoclinic to tetragonal to cubic zirconia were observed; however, in gas-phase derived zirconia ceramics two cubic phases were observed at about 1200 degrees C. Most of the grains (90-95%) contained 8 mol% Y2O3, the expected composition, whereas the minority cubic phase, which disappeared on further heating, contained about only 3 mol% Y2O3. For samples sintered at 1500 degrees C, bulk conductivities, measured at 1000 degrees C, were 5.5 Sm-1 and 10 Sm-1 for co-precipitated and gas-phase derived samples, respectively. Both exhibited a similar activation energy and showed little evidence of grain boundary resistance above 500 degrees C. The influence of phase development sequence upon electrical properties and structural stability is discussed.