Gasoline-derived deposits in modern automobile engines can plug fuel injectors and impair engine performance. The deposits (which originate from gasoline oxidized at moderate temperatures) contain higher sulfur concentration than the fuel. We used laboratory tests to study the effect of organic sulfur on the formation of solid deposits in an uninhibited gasoline oxidized at 100°C. Gravimetric studies showed that the effect of sulfides on deposit formation was strongly dependent on the structure of the sulfur compound. Disulfides and thiophenes showed little or no effect. Thiols enhanced the formation of insoluble deposits; but 1-dodecanethiol decreased the amount of deposits formed after long oxidation times. Analysis of thin deposit films with XPS(ESCA) showed that sulfur in the deposits was primarily bound to oxygen. When the fuel was blended with thiols, non-oxidized sulfur also appeared in the deposits. The effect of the tested sulfur compounds (with the exception of thiols) is probably due to interference with the fuel's oxidation mechanism rather than direct interference with the deposit formation steps. Sulfur in the fuel is concentrated in the high molecular weight components of the gasoline, and these components act as the original precursors for the formation of insoluble deposits.