A thick film solid state electrolyte on an alumina substrate has already been applied for fabrication of oxygen sensors. The results of our investigations indicated that the purity of applied alumina substrate influences the electrical characteristics of the sintered, thick film, cubic ZrO2 solid electrolyte. After firing of the ZrO2 layer at 1450-degrees-C only the cubic modification was present on the 99% alumina substrate. However, on 96% alumina one third of the ZrO2 was present in the monoclinic modification. Microstructural analysis of fired samples shown that yttria diffused into the silicate glassy phase which is added as a sintering aid to 96% alumina. Due to partial phase transformation of cubic zirconia to the monoclinic modification, a significantly higher dc resistance of sintered films was measured on 96% alumina substrates.