A fuel composition comprising a major proportion of a base fuel, preferably a diesel fuel, and a minor proportion of a polymeric additive, which additive is soluble in said base fuel and is present in an amount sufficient to impart to the composition a kinematic viscosity which is greater than that of said composition not comprising said additive; a method of operating a compression-ignition engine, which comprises introducing into the combustion chambers of said engine said fuel composition; a method of improving the ability of a compression-ignition engine to start when at elevated temperatures by employing said fuel composition; and the use in a compression-ignition engine of said fuel composition to improve the ability of the engine to start when at elevated temperatures.
