A lock-in amplifier and linear ramp generator system has been developed which permits an rms temperature resolution of 3-4 m°C. This temperature resolution approximates the theoretical Johnson noise limit of the signal source. The system was employed to improve the sensitivity of thermometric analysis. In a 100-ml calorimetrie vessel, solutions of 15 nM perchloric acid have been titrated with sodium hydroxide with a precision and accuracy of better than 10%. This reaction generated 20 millicalories of heat and caused a temperature change of approximately 0.2 m°C. At higher concentrations (1.5mM) the optimum precision achieved was 0.2%, which is the limiting precision of the volume delivery device.