Comparative impedance measurements of electric conductivity of nominally undoped yttria are carried out for samples with sharply differing morphologies, namely, single crystals and porous ceramics (with a density of 67%) over a 980–350°С temperature range in air atmosphere of different humidity (рН2О= 40–3000 Pa). Dilatometric analysis of possible structure changes is carried out. The effects of uncontrolled impurities on the yttria defect structure are analyzed.