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