A micro-bomb oxygen combustion calorimeter suitable for samples with masses of about 20 mg is described. In order to prevent incomplete combustion, a small heater is fitted under the crucible. The calorimeter was tested by determining the standard molar enthalpy of formation of salicylic acid in the crystalline state at T = 298.15 K: Delta(f)H(m)(o)(cr)/(kJ.mol(-1)) = (588.4 +/- 1.7).