In India, during 1992, two wheelers numbered around 17 millions compared to only 2.8 million passenger cars. Two wheelers are estimated to consume about 60% of the total gasoline annually. Very little information however, is available on the octane number requirement of two stroke engine two wheelers.In this study, octane number requirement (ONR) of 13 makes of two wheeled vehicles constituting the bulk of current production was measured using full boiling range gasolines as well as primary reference fuels. Knock intensity was measured by an instrument which measures and analyzes cylinder head vibrations in the frequency range of 5 to 12 kHz. Among different makes, ONR varied from 51.5 to 101 research octane number (RON) and 48-86 motor octane number (MON). ONR of the 2-stroke engine powered vehicles measured using primary reference fuels was found to be closer to MON of the full boiling range reference fuels.OCTANE NUMBER REQUIREMENT (ONR) of passenger cars has been studied regularly in the United States by the Co-ordinating Research Council (CRC) and others (1, 2, 3, 4 and 5). Similar studies however, are not available for mopeds, motorcycles and scooters. These vehicles consume insignificant quantities of total gasoline in the USA and Europe and perhaps their octane requirement does not have much bearing on the marketed gasoline quality and petroleum refining processes.Many two wheelers have been traditionally powered by small, air-cooled, 2-stroke engines. Most studies (6, 7, 8 and 9) involving these engines have related to an understanding of abnormal and knocking combustion and not on the measurement and projection of the octane number requirement of 2-wheeler population. In one study (6) the octane number requirement of an outboard 2-stroke engine was measured during development of a new combustion chamber. Besides this, very little published information is available on the subject.The two wheelers in India have a very large population and consume more gasoline than the passenger cars. The present work was aimed to evaluate ONR of some maker of two wheelers used commonly in this commonly. In the present paper, results of this study are reported.