Certain UV filter compounds improve the combustion properties of diesel fuel compositions, and in particular modify the ignition delay and/or modify the burn period and/or modify the peak pressure and/or increase the cetane number of the liquid fuel composition. A liquid fuel composition is provided containing: (a) a diesel base fuel suitable for use in an internal combustion engine; and (b) in the range of 10 ppmw to 2 wt %, by weight of the liquid fuel composition, of one or more organic UV filter compounds selected from imidazoles, triazines, triazones and triazoles, and mixtures thereof.