Washcoat sintering and substrate meltdown have traditionally been the principle deactivating mechanisms of catalysts fitted to two-stroke engines. The reduction of the excessively high HC and CO levels responsible for these effects has therefore been the focus of considerable research which has led to the introduction of direct in-cylinder fuel injection to some larger versions of this engine. However, much less attention has been paid to the effects of oil and its additives on the performance and durability of the two-stroke catalyst. The quantity of oil emitted to the exhaust system of the majority of two-stroke engines is much greater than in four-stroke engines of comparable output due to the total loss lubrication system employed. The fundamental design of the two-stroke also permits some of this oil to ‘short-circuit’ to the exhaust in a neat or unburned form. The effect of this oil on the catalyst and the subsequent effect of the catalyst on the oil is the subject of this paper.Two separate experimental test facilities were used during this investigation. The first was a computer-controlled catalyst test rig which uses a synthetic exhaust gas to examine the light-off characteristics of the catalyst in an accurate and repeatable manner. The second was based on a propane gas burner and was used to subject the catalyst to oil under conditions representative of those experienced on an operating engine.Two oil types were formulated specifically for this study, one containing a minimal sulphur concentration and one containing 1.02% sulphur. It was found that oil with a low sulphur content caused only temporary deactivation which could be removed by simply heating the catalyst while oil which contained a high level of sulphur caused immediate and permanent deactivation. It was also found that the temperature at which the oil was layered onto the catalyst had a significant effect on the post-oiling light-off performance with CO conversion being inhibited to a greater degree than HC conversion. BET surface area measurements revealed a significant reduction in the number of micropores in the 5-10Å region while surface reaction rate considerations suggested a loss in metal surface area of over 50%.