The Er2+xTi2-xO7-delta (x = 0.096; 35.5 mol% Er2O3) solid solution and the stoichiometric pyrochlore-structured compound Er2Ti2O7 (x = 0; 33.3 mol% Er2O3) are characterized by X-ray diffraction (phase analysis and Rietveld method), thermal analysis and optical spectroscopy. Both oxides were synthesized by thermal sintering of co-precipitated powders. The synthesis study was performed in the temperature range 650-1690 degrees C. The amorphous phase exists below 700 degrees C. The crystallization of the ordered pyrochlore phase (P) in the range 800-1000 degrees C is accompanied by oxygen release. The ordered pyrochlore phase (P) exists in the range 1000-1200 degrees C. Heat-treatment at T >= 1600 degrees C leads to the formation of an oxide ion-conducting phase with a distorted pyrochlore structure (P2) and an ionic conductivity of about 10(-3) S/cm at 740 degrees C. Complex impedance spectra are used to separately assess the bulk and grain-boundary conductivity of the samples. At 700 degrees C and oxygen pressures above 10(-10) Pa, the Er2+xTi2-xO7-delta (x = 0, 0.096) samples are purely ionic conductors. (c) 2006 Elsevier Ltd. All rights reserved.