This document specifies a procedure for rating the tendencies of gas turbine fuels to deposit decomposition products within the fuel system. It is applicable to middle distillate and wide-cut fuels and is particularly specified for the performance of aviation gas turbine fuels. The test results are indicative of fuel thermal oxidation stability during gas turbine operation and can be used to assess the level of deposits that form when liquid fuel contacts a heated surface at a specified temperature. This method is also applicable to aviation turbine fuel that consists of conventional and synthetic blending components as defined in the scope of for instance ASTM D7566[1] and Def Stan 91-091[2]. NOTE For the benefit of those using older instruments, non-SI-units and recalculated numbers are given in between brackets where they are more suitable.