The kinetics of thermal dehydration of microcrystalline powders and single-crystals of barium chlorate monohydrate were studied by differential scanning calorimetry. The computer elaboration of thermal data showed that a single kinetic law could not account for the whole decomposition; moreover, different laws had to be used for powders than for single-crystals, whose reaction appeared diffusion-limited towards the end of the dehydration. Optical microscopy was extensively used as an independent source of information to clarify some of the thermal results and to gain a deeper insigth into the possible reaction mechanism.The use of thermal methods to study the reactivities of solids has become more and more popular because of the relatively easy experimentation. Parallel to this growth, a deep improvement of the underlying theory has taken place [1 ]. However, the deduction of meaningful kinetic parameters is not always as straightforward as it might appear and a careful analysis of the thermal data, usually performed by computer techniques, is necessary. Nevertheless, this is sometimes not enough and an independent source of information is required.Optical microscopy is particularly useful when reactions of the type Asoli a Bsoli a + Cgas , in which nucleation takes place on external surfaces, are considered; in fact, nucleation and growth rates are easily obtained. Moreover, if correspondence of nuclei and sites of emergence of dislocations is evidenced, some insight into the reaction mechanism may be gained by a determination of the possible dislocation systems in the parent crystal, using the etching technique.In a study on the kinetics of thermal dehydration of a number of crystal hydrates and hydroxides, planned to compare structure, defects and reactivity in order to seek possible correlations, barium chlorate monohydrate (BCM) was taken into consideration because a) its structure is known; b) crystalline powders and single-crystals are easily obtained; c) the water molecule is lost stoichiometrically in a single step; d) its crystals are easily cleaved.Moreover, preliminary thermal runs had evidenced some discrepancies in the behaviour of powders and single-crystals which were not amenable to the effects of the parameters which usually influence the thermal curves (e.g. sample mass, nitrogen flow, scan speed) [2]. BCM therefore appeared particularly suitable to check the complementarity of thermal methods and optical microscopy.