The effect of Mn on the structure, lattice parameter and conductivity has been investigated for near-cubic YSZ with an yttrium content slightly under 8 mol% Y2O3. The structure and chemistry of the material were studied both as sintered, and also after prolonged heat treatments at 850 and 1000 degreesC, using x-ray diffraction, electron microscopy, impedance spectroscopy and d.c. conductivity measurements. In the latter case, the measurements were made in-situ during ageing. At 1000 degreesC and for Mn additions above about 2 mol% YSZ is found to be stabilised as the cubic structure, and thereby the conductivity is also stabilised. However, at all temperatures studied, the presence of Mn considerably reduces the ionic conductivity of YSZ.