The effect of both soluble gum and insoluble particulate gum from the ageing of automotive diesel oil (ADO) has been investigated. The standard experimental procedure has been shown to be a limiting factor in the precision with which soluble gum in diesel fuels may be measured. Greater reproducibility was obtained by extending the steam jet bath heating period to two hours. Filtration characteristics were determined for both laboratory and automotive filtration media with particulate gum from naturally aged fuel and inorganic particulate matter from AC Fine Test Dust. The filtration characteristics were shown to be dependent both on the type of filtration medium and the properties of the particulate matter.