Automatic equipment based on optical detection has been used for petroleum products successfully. Examples include ASTM D5773 Standard Test Method for Cloud Point Determination of Petroleum Products and ASTM D5972 Standard Test Method for the Freezing Point of Aviation Fuels. Recently it has been shown that the technology can be extended to engine coolants1.The determination of freezing point in aqueous ethylene glycol based systems is imperfectly understood and complicated by non-equilibrium solidification. The current ASTM D 1177-94 method includes manual seeding to overcome this but requires a cryogenic bath, time and considerable operator skill. Experimentally determined aqueous ethylene glycol freezing points show good agreement between laboratories only to approximately 60 wt %. In the 58 wt % to 80 wt % region, one, two and even three eutectics have been reported at varying temperatures. It is likely that this is due to the tendency of the mixture to super-cool in this concentration range. Modern coolant manufacturers and users typically assume a single eutectic in their specifications.In this work, data collected using an automatic optical sensor based freeze point detector are compared and contrasted to results with the ASTM D 1177 method and literature values. The operation of the automatic analyzer and its algorithm are discussed. Variables including concentration and coolant chemistry are investigated. Statistical analysis indicates that the freeze point values determined by the automatic method are more accurate and sensitive than the manual method in the examined ranges. Preliminary results support the single eutectic finding.