Polar Heteroatomic species have been correlated with incompatibility reactions in middle distillate fuels. Instability is defined as the formation of filterable sediments and fuel insoluble gums. Heteroatoms (oxygen, nitrogen and sulfur) have been found to be greatly enriched in such sediments. Trace levels of certain organo-sulfur compounds, especially sulfonic acids in model fuel systems, have been found to significantly increase deposit formation. The effectiveness of organic amines as additives suggests that acid/base chemistry is also involved in the formation of deposits. This paper reports on a study of p-ethylbenzene sulfonic acid reacting with 3-methyl indole in model systems. A possible reaction mechanism and a proposed structure for the product is reported. The results of the model system are contrasted to results from previous actual fuel systems.