Organic acids promote the formation of insolubles in middle distillate fuels, particularly in blends containing cracked stock. Studies with a sulfonic acid demonstrate that this class of acids has a large effect and that it participates in two roles. First, it acts as an acid catalyst on the basis of the hydrogen ion concentration. In the other mode, sulfonic acid participates directly in insolubles formation, probably by salt formation with organic bases in the fuel. This latter suggestion is supported by the findings for nonaqueous titrations forstrong acids and findings of XPS and mass spectral analyses. Thiols also increase insolubles formation. A part of the thiol oxidizes to sulfonic acid, which can act in two roles:(1) as an acid catalyst for insolubles producing reactions and (2) as a direct participant in insolubles formation. The mechanism of interaction of chloroacetic acid has not been defined.