In continuation of our study of the thermal-oxidative degradation of lubricants using PDSC, we investigated the biddegradable metal-working and hydraulic fluids that are available on the European market. Isothermal onset times of oxidation were measured at different sample temperatures and plotted against the reciprocal temperatures, giving straight ageing lines which were used to differentiate between the thermal-oxidative stabilities of the oils. The stabilities of metal-working oils and hydraulic fluids vary over a wide range. Synthetic ester oils are more stable than vegetable-based fluids; however, our work demonstrates that the latter can be improved by selected antioxidants to yield equal or even better thermal-oxidative stabilities. Measurements conducted on steel surfaces show a strong catalytic influence compared to an inert aluminium (Al2O3) surface for both fluids. We also investigated the stabilities of laboratory-aged hydraulic fluids (ASTM-D-2893). The results, by PDSC alone or in combination with conventional oxidation tests, show that the ageing behaviour of biodegradable lubricants can be assessed effectively. Commercial products are by no means of equal quality in this respect. In our opinion, single PDSC measurement could offer many advantages over the conventional oxidation tests used, such as the Baader test (DIN 51554) or the Rancimat test. Typical results for commercial products based on rapeseed oil and synthetic esters are presented.