Microflow calorimeter design for heats of mixing is surveyed. Precise temperature equilibration of the fluids before mixing, the elimination of frictional energy effects from the instrument response, and the absence of flow‐rate and physical‐property‐dependent heat leaks are particularly important. A design using a differential mode of operation is based on a careful analysis of entropy generation in flows with mixing and friction. The compensation for frictional heating and the lack of dependence of measurements on flow rate and mixture physical properties other than hE represents a major advance. Measured data for the welltested cyclohexane‐hexane system had an average deviation of only 0.41%. For the more viscous cyclohexane‐1‐hexanol system, measured data of similar precision are considered the best available.