The effect of the velocity of application of foam to the surface of flammable liquid fires has been assessed, using control and extinction times as the performance criteria. Four different types of foam liquid were used on one hundred and thirty 0.28 m2 fires in petrol and AVTUR fuels. Extinction times were markedly affected by application velocity; control times to a lesser extent. Large differences in behaviour were found between the different types of foam liquids, and between different batches of protein foam liquid. These differences were found to depend on the type of fuel. High application velocity was not found to cause breakdown of the foam, but to produce mixing of the fuel and foam, thus preventing extinction. The higher expansions and shear stresses favoured effective extinction at the higher application velocities. The implication of the results to the design of laboratory test equipment and methods is discussed.