The potential use of vegetable oil derived industrial oils continues to be of great interest because vegetable oils are: relatively non-toxic, biodegradable and they are a renewable basestock alternative to petroleum oil. However, the fatty ester components containing conjugated double bonds of the triglyceride structure of vegetable oils typically produces considerably poorer thermal oxidative stability than that achievable with petroleum basestocks under typical use conditions. This is especially true when a vegetable oil is held in an open tank with agitation and exposure to air at elevated temperatures for extended periods of time (months or years). Furthermore, when used as quenchants, furnace loads of hot steel (850 °C) are typically rapidly immersed and cooled to approximately 50-60 °C bath temperatures  for steel hardening applications. Clearly, for this application, reasonable thermal-oxidative stability is essential. This paper will review thermal-oxidative stability and quenching performance of a series of vegetable oils representing a range of compositions. Studies conducted to compare the hardnesses and microstructures of AISI 1045 and 4140 quenched into soybean oil and palm oil will be discussed.