Ethylene glycol based engine coolants are increasingly being judged as hazardous wastes. Across the USA, states are beginning to disallow these materials in their sewage systems (1)*. Various methods are being used to allow reuse of these coolant materials. Purification by vacuum distillation is one process that produces a very pure ethylene glycol for reuse. This paper describes the physical chemistry of the vacuum distillation process as it applies to an aqueous solution of ethylene glycol contaminated by a multitude of dissolved and insoluble solids. It summarizes data that shows the resultant ethylene glycol is of excellent quality for producing stable and protective engine coolant.The science of distillation has been used for centuries for purifying volatile materials; in recent decades, distillation processes have been developed that result in extremely high levels of purification. In general terms, many modern-day chemicals (including ethylene glycol) are purified during their production by distillation. Thus, distillation is involved with many chemical processes and is a proven, well developed method of separation.The fundamentals of the distillation process involve the heating of a multi-component mixture to the point where the most volatile component(s) will vaporize (often called “boiling”). The vapor, of course, is richer in the more volatile component(s) than the remaining liquid. In batch distillation, the vapor is cooled and condensed back into a liquid condensate enriched in the most volatile component. In the more common and complex fractional distillation, the batch process is, in essence, repeated over and over again by special, expensive equipment to gain a greater degree of separation. Generally tall fractionating columns for multiple reboiling-recondensing using packing or “bubble caps” are used. Nearly perfect separation of each volatile component is, in some cases, possible with this equipment. When two materials, as in used engine coolants, have widely differing boiling temperatures, a single stage distillation process is quite effective - as shown in this paper.