The refractive index of ethylene and propylene glycol-based coolants is directly proportional to the concentration/freezing point of coolant in aqueous solution. A complicating factor in concentration measurement by refractive index is the temperature dependence of this physical property. The degree of change in refractive index due to changes in temperature is also concentration dependent. Accurate measurements require temperature equilibration between the refractometer, sample and environment. A refractometer which is able to sense temperature and automatically apply a correction factor greatly increases accuracy.Data showing the effect of temperature on a range of concentrations of commercial coolants aid pure ethylene and propylene glycol solutions is presented. The method and means of automatic temperature compensation necessary for obtaining the highest degree of accuracy are described.