A method for increasing the cetane number of a fuel composition containing a Fischer-Tropsch derived fuel component, in order to reach a target cetane number X, is provided by adding to the composition a concentration c of an ignition improver, wherein c is lower than the concentration which theory would predict needed to be added in order to achieve the target. The ignition improver is preferably 2-ethylhexyl nitrate and the fuel composition suitably a diesel or kerosene fuel. A fuel composition for use in a compression ignition engine, which has a cetane number of 85 or greater, and contains a Fischer-Tropsch derived fuel component and an ignition improver is also disclosed.