This paper presents a comprehensive study on the influence of different high octane fuels (such as eucalyptus oil, ethanol and methanol) on combustion behaviour of a Mahua oil based dual fuel engine. A single cylinder diesel engine developing a power output of 3.7 kW at 1500 rpm was modified and tested under dual fuel mode of operation. Initially the engine was run using neat diesel (ND), neat Mahua oil (NMO) as fuels. In the second phase, the engine was operated in dual fuel mode by using a specially designed variable jet carburettor to supply the high octane fuels. Engine trials were made at 100% and 40% loads (power outputs) with varying amounts of high octane fuels up to the maximum possible limit. Results indicated significant improvement in combustion pressure and maximum rate of pressure rise in dual fuel operation with all the inducted fuels mainly at 100% load. The ignition delay was found to be higher for all the inducted fuels as compared to single fuel operation for both loads. The combustion duration reduced in dual fuel operation as compared to NMO operation at 100% load but increased at 40% load. Significant improvement in heat release rate was observed in the premixed combustion phase in dual fuel operation as compared to single fuel mode of operation. The optimal energy share for the best combustion behaviour was found to be 44.6%, 27.3% and 23.2% respectively for eucalyptus oil, ethanol and methanol at 100% load. Eucalyptus oil showed the best among the inducted fuels in raising the cylinder peak pressure and heat release rate with reduced ignition delay and combustion duration. In general, dual fuel operation showed inferior combustion behaviour at 40% load for all the inducted fuels. © 2017