An aneroid high precision semi-micro reaction calorimeter is described. It may be used to investigate the calorimetry of a wide range of reacting systems including those in which one component is a gas. Its heat capacity is 120 cal degC^-1 and, in conjunction with the ancillary monitoring equipment, heat outputs of the order of 10 cal may be measured with a precision of ±0.2%. Temperature changes are followed using a 75 c/s bridge incorporating a novel low noise phase-sensitive null detector and the temperature against time data are automatically recorded on a print-out counter.