This paper describes the design, development, and construction of an in-line pyrolysis unit for the mass spectrometric study of the thermal stabilities of light oils or volatile liquids. A sealed glass ampoule, containing about 100 mug of sample, is dropped into the furnace which is preheated and controlled at the desire d temperature. After the chosen pyrolysis time has elapsed, a piston is screwed down to break the ampoule, and the pyrolysis products are thereby released and impelled by helium carrier gas via a heated capillary into the mass spectrometric source. For gas chromatography-mass spectrometry (GC-MS) analysis, the products pass directly through the GC column before entering the MS source. The glass fragments are contained within a perforated tube, which is removed, cleaned, and replaced before inserting a new ampoule. This new pyrolysis unit has small dead volume, is precisely temperature-controlled, and contains no cold regions where products would condense. Results obtained with this equipment show that the pyrograms have good resolution, and because there are no product losses, the available sensitivity is high. The use of this unit is demonstrated by measuring the thermal degradation of two types of base oil at 400 degreesC for various durations of heating, and analysing the products by GC-MS. The unit can also be used to study the thermal behaviour of oils in different atmospheres or with specific additives. (C) 2002 Elsevier Science Ltd. All rights reserved.