The structure and the microstructure of nickel manganite NTC thermistors have been determined and correlated with their electrical properties in order to understand the 'ageing' phenomenon observed in these semiconductor devices. It has been shown that the heat treatment of metallization by serigraphy is responsible for structural and microstructural modifications. During this annealing some of the Ni2+ cations migrate from the octahedral sites to the tetrahedral sites. At the same time, the ceramic is oxidized and precipitates of a cationic defect spinel phase appear. These modifications lead to a decrease in resistivity. During the ageing test at 125 degrees C the cationic distribution does not change but the precipitates disappear. It induces an increase in resistivity. An improved serigraphy process is proposed to achieve stable NTC thermistors.