The continually increasing operating temperatures of modern jet engines, and their demands on lubricants, made it obvious some time ago that the 7808-type of synthetic fluid would not be adequate for Mach 2 and Mach 3 aircraft. This realization led to the publishing of target specification Mil-L-9326A, which called for a 100-hr engine test with a bulk fluid temperature of 400 F. The struggles involved in industry's three-year search for suitable base materials are well known. It now appears that such products can be supplied by using polyhydroxy alcohols and monobasic acids, as opposed to the 7808 types of materials, which used monohydroxy alcohols and dibasic acids. Specifically, suitable materials have been prepared by esterifying trimethylolpropane and straight-chain acids of the proper chain length. Chain length, of course, has a direct influence on the viscosity of the resulting ester. In addition, mixtures of acids can be employed in the esterification to give any desired average chain length. Typical viscosity curves superimposed on the Mil-L-9326A requirement are shown in Fig. 1. Also shown as information are esters of pentaerythritol and neopentyl glycol. While there may well be other suitable base materials, our company, because of the raw material situation, concentrated its efforts on the trimethylolpropane and pentaerythritol esters, and this paper deals solely with these materials. Having determined that the trimethylolpropane esters had viscosity properties closest to those desired, we will now consider the thermal and oxidative properties. Initial testing done in our laboratories was of a screening nature and was intended to establish primarily the thermal stability of these esters. This consisted of putting the esters in a stainless steel bomb, sealing at atmospheric pressure, and heating the bombs to 600 F for 48 hr. This was the first test that indicated the potential for the trimethylolpropane ester. Since we are not interested here in routine property discussions, we will omit a detailed properties analysis. Properties for such an ester are shown, however, in Table I. It is of interest to note that extreme-pressure, load-carrying capacity of the base ester is not as high as originally called for in the target specification. However, modifications in the Ryder gear testing procedure, involving testing at 400 F, and the development of new gear materials have considerably changed the picture as to what will actually be required. Based on the successful engine testing done thus far, it appears that the trimethylolpropane ester will be more than satisfactory without extreme-pressure additives.