Air-dried 60 mg samples of a German brown coal were examined using a microcalorimeter at temperatures in the range 30-55 degrees C. Heat release rates in the range similar to 1-25 mu W were recorded, and an irreversible effect was observed whereby once the sample had experienced a temperature of similar to 42 degrees C, previously obtained results at lower temperatures could not be repeated. This is believed to be due to chemisorption.