The preignition processes in a dual fuel engine are described and the roles of the formation and consumption of formaldehyde in these and subsequent processes are discussed. Reference is made to the results of detailed chemical kinetic modelling of the oxidation reactions of the gaseous fuel component during the compression stage. This is supported by experimental evidence of the kinetic role of formaldehyde through its deliberate induction with the intake charge of a dual fuel engine over a range of operating conditions and fuels. It is suggested that the preignition reaction activity of the gaseous fuel-air charge during compression contributes significantly to the observed extension of the ignition delay in dual fuel engines at very low load conditions when relatively small gaseous fuel concentrations are being used. It is also shown that the deliberate changes in the concentrations of formaldehyde within the charge during compression can modify the rates of the preignition reactions of the diesel pilot and hence the ignition delay significantly.