Abstract: A method for increasing the cetane number of a diesel fuel composition which contains a major proportion of a diesel base fuel is provided, in order to reach a target cetane number X. A fatty acid alkyl ester (FAAE) having a cetane number B greater than the cetane number A of the base fuel is added to the base fuel in an amount x, wherein x is less than the amount of the FAAE which would need to be added to the base fuel in order to achieve cetane number X if linear blending rules applied. The concentration of the FAAE in the overall fuel composition is preferably from 0.05 to 25% v/v.