Furnace tube fouling by burning of residual fuel oil (RFO) is widely encountered in both process as well as boiler furnaces. Fouling becomes significant when fuel contains sulfur compounds and metallic constituents such as V, Na, Ni, Fe etc. Study on one such problem, faced in an Indian refinery has been taken up. In the present study, furnace deposits collected during shut-down operations of atmospheric distillation unit, have been analysed using XRD, ICAP, XRF and IR techniques. Factors responsible for frequent fouling of process furnace tubes were identified. Sodium nickel sulfate formed during combustion process causes primarily the formation of deposits on furnace tube surfaces. For reducing furnace tube deposit, different types of commercially available additives have been evaluated. Incorporation of calcium based additives modified the deposit into non-sticky high melting phases.