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The thermal decomposition of a series of nontronites and a ferruginous smectite has been studied using a combination of high-resolution thermogravimetric analysis with a controlled heating rate and the mass spectrometry of evolved water vapour. The temperature-mass spectrum curve follows the differential thermogravimetric curve with precision, thus providing a second means of calculating the percentage weight loss. Three dehydration steps were observed in the CRTA experiment at around 86, 117 and 245°C depending on the nontronite sample. Two overlapping dehydroxylation steps were observed at approximately 350 and 400°C with a weight loss ratio of 1:2. A comparison is made between the between the results obtained from the dynamic differential thermogravimetric experiment and the controlled rate thermal analysis experiment. Key words: nontronite, ferruginous smectite, controlled rate thermal analysis, differential thermogravimetric analysi