The advent of flexible fuels has created a need in the automotive industry for materials that perform in the exceptionally aggressive environment of alternate fuels.This Paper outlines an extensive alternate fuel testing study by Hoechst Celanese of five different crystalline engineering thermoplastics including Acetal Copolymer, Nylon 6/6, Polyphenylene Sulfide, PBT Polyester, and Liquid Crystal Polymer. These five base resins were used in unfilled, glass fiber reinforced, impact modified, glass/mineral reinforced, and long–glass fiber reinforced grades. All the materials were tested in Fuel C, Auto–Oxidized (sour gas), and Aggressive Fuel with M25, M50, M85 at 60°C and 121°C.The driving force motivating Hoechst Celanese to evaluate their crystalline engineering thermoplastics is the automotive industries shift towards alternate fuels and higher operating temperatures. This was reinforced by the SAE cooperative research program as stated below: “Materials for Use with Methanol (per ‘Gasoline/Methanol Mixtures for Materials Testing’. An SAE Cooperative Research Report, September 1990.) Both the California and Federal Governments have passed or are considering legislation mandating the use of methanol as an alternative fuel. This activity has instigated development programs leading to automobiles capable of operating on oxygenated fuels. However, there is limited practical knowledge regarding the performance and durability of vehicle systems with such fuel. To meet these challenges, Chrysler Corporation, Ford Motor Company and General Motors Corporation formed an Oxygenated Fuels Task Force in accordance with the Cooperative Research Act of 1984. This Task Force is comprised of four groups. The specific scope of Project Group 2 was to develop and exchange information relative to materials and test methods for use with blends of methanol and gasoline. When considering the evaluation of materials for use in gasoline and gasoline/methanol mixtures, the question of appropriate test fuels is both an obvious and immediate concern. Because of the variability in fuel composition between manufacturers and batches from the same manufacturer, as well as systematic changes for altitude and prevailing weather conditions, the use of real fuels poses unacceptable variability problems. Further, fuel grade methanol may contain more than just methanol. To address this issue, the SAE Cooperative Research Project Group 2 arrived at a series of test fuels which should be used in comparing various materials to determine suitability for use in the broad spectrum of compositions ranging from gasoline to M85 (85% methanol). ASTM reference fuel “C” (50% toluene, 50% iso–octane) was chosen as a surrogate for gasoline. The high aromatic content provides a fuel which is aggressive for swelling polymeric materials. Because it was anticipated that the methanol actually seen in the field would contain more water than is allowed by the definitions for reagent grade methanol, distilled water was added. Various possible contaminants in an actual gasoline can aggravate material deterioration. In addition to the basic set of fuels, aggressive fuels were needed to challenge potential fuel system materials. Some of the components in gasoline can decompose by a process called auto–oxidation to form aggressive substances which can attack polymers and corrode metals. The formation of free radicals from hydroperoxides is accelerated by the presence of trace metals, such as copper, in the fuel. Therefore, when determining the effect of auto–oxidized fuels on polymers, controlled amounts of ions and hydroperoxide were added to the test fuels. The test fuels for polymeric materials minimize testing while leading the engineer to potentially suitable candidate materials. Of course, the choice of final material to be used is dependent on the application as well as the specific design and involves the cooperative efforts of the component engineer, materials engineer and suppliers.” [The SAE Cooperative Research Project Group 2 was formed by the Oxygenated Fuels Task Force, which is composed of OEM automotive engineering executives. Their task is to identify and prioritize potential areas for pre–competitive cooperative research programs to operate under the administration of SAE. The specific scope of Project Group 2 is to develop and exchange information relative to materials and test methods for use with blends of methanol and gasoline. Hoechst Celanese Engineering Plastic Division tested a number of their engineering resins which are used in fuel systems according to the recommendations and procedures outlined by the SAE Cooperative Research Project Group 2.] This paper summarizes the results of the various types of crystalline engineering thermoplastics when subjected to alternate fuels and elevated temperatures. This data will be utilized as guidelines to determine the appropriate type material for the appropriate application.