A precision isoperibol calorimeter with automatic data acquisition and processing, capable of operating over a wide temperature range, has been constructed and tested. The calorimeter was designed to have a very low thermal modulus and fast equilibration time. A small vapor space above the liquid and a vapor trap make the calorimeter useful for measurements with nonaqueous solvents. Time–temperature data are automatically recorded in digital form on magnetic tape and processed on a Univac 1106 computer. A series of electrical calibrations causing a temperature rise of 0.13 K were reproducible to about 0.01%. Several determinations of the enthalpy of reaction of a standard sample of Tris (hydroxymethyl) aminomethane with 0.1NHCl gave an average value of 29.764±0.013 kJ/mol, which agrees satisfactorily with published results.