Temperature-programmed reduction (TPR) is a valuable tool for the characterisation of catalysts. The reductant mostly used is hydrogen. Hydrogen reduces the catalyst by producing water. The amount of hydrogen reacted is measured by menas of a thermal conductivity detector. In this paper the temperature-programmed reduction of vanadia and molybdena catalysts by ammonia is emphasized. During the temperature-programmed reduction ammonia is consumed. The compounds formed in this process are detected by means of a mass spectrometer. It was found that almost exclusively N2, H2O and H2 were formed. Hydrogen formed by the decomposition of ammonia over vanadia and molybdena above 600° produces hydroxyl groups, which are a source for the formation of water at temperatures above 600°.