A new method for the measurements of reaction enthalpies based on a laser deflection heat-sensor was developed. The heat released or absorbed from a chemical reaction in an aqueous phase causes the formation of thermally driven refractive index gradients in an adjacent organic phase. This effect is monitored by the change in the deflection angle of a laser light in the organic phase. The instrument was tested in the complexation reaction of EDTA with magnesium (endothermic) and calcium (exothermic). It is shown that the direction of the deflection angle is indicative of the sign of the enthalpy change. The magnitude of this enthalpy change can be calculated using the intensity of the maximum deflection and a calibration slope obtained using the reaction enthalpy of the NaOH/HCl neutralization. The heat limit of detection (HLOD) was calculated to be 60 mJ. Possible improvements in the HLOD are discussed.