Calcium hydroxide particles deposited on substrates have been investigated by thermal analysis. Dehydration was successfully detected by DSC (heating rate, 1-20-degrees-C min-1) for the specimen (thickness about 0.80-2.57 mum) as-deposited on the glass substrate (5 mm x 5 mm x 0.15 mm). The peak temperature and the heat of dehydration (278 +/- 10 cal g-1) was independent of the thickness in the present study. The deviation from the thermochemical data (328 cal g-1) was explained semi-quantitatively as being due to CaCO3 impurity; this hypothesis was supported by the TG-DTA of the deposit removed from the bottom of the vessel.