The deterioration of automotive diesel oil (ADO) in storage has been investigated. The quality of the fuel produced by the refinery was shown to be the major factor determining the amount of chemical degradation which will occur. ADO containing only distillate from the primary distillation unit is a stable fuel. Blending unhydrogenated catalyticaliy cracked or thermally cracked refinery light cycle oil (LCO) with the fuel substantially decreased the storage stability. The amount of organic particulate matter formed on storage is dependent upon both the chemical constitution of the LCO and the fuel with which it was blended. Chemical additives which are used to reduce soluble gum formation in gasoline were found to be ineffective in ADO. Particulate formation in ADO was reduced by additives which inhibited acid catalysed processes in the fuel.