Wishing to extend its possibilities, we have used the CRMT rocking calorimeter in association with a microbomb to determine the enthalpy of combustion of a sample of low mass (about 5 mg) of a substance containing chlorine. p-Chlorobenzoic acid, p-ClC6H4COOH, a candidate as reference material for combustion calorimetry of substances containing chlorine, has been investigated. The value - (3063.3 +/- 1.5) kJ . mol(-1) for its molar enthalpy of combustion obtained with this apparatus is in good agreement with the authors’ and literature values obtained by classical combustion calorimetry.