Abstract--The thermal decomposition of a kaolinite:dimethyl sulfoxide intercalate has been studied by thermogravimetry. The decomposition is complete by 175~ and has a AH of 58 • 5 kJ/mole of dimethyl sulfoxide. For the first 70-80% of the reaction a plot of log(1 -a) vs. time is linear, where c~ is the reaction fraction complete. A two-dimensional, contracting-circle type of mechanism is consistent with this linearity, provided that the nucleation process is considered to be exponential rather than instantaneous. An activation energy of 105 • 8 k J/mole of dimethyl sulfoxide has been derived. This value is inconsistent with the simple assumption that the rate-determining step is the freeing of single organic molecules from their bonding to the clay layers. The mechanism of this decomposition is different from that found for kaolinite: N-methyl formamide, and the activation energy is considerably greater than that found for that material (30 • 3 kJ/mole).