Autoignition of turbulent methane/air mixing layers, in which n-heptane droplets have been added, was investigated by DNS. This configuration is relevant to dual-fuel, pilot-ignited natural gas engines under direct injection conditions. Two passive scalars were introduced in order to describe the dual fuel combustion. It was shown that the pre-ignition phase is dominated by n-heptane oxidation while methane oxidation is less intense. During the pre-ignition phase the methane/air mixing layer is distorted due to turbulence creating regions around the n-heptane droplets allowing the transport of intermediate species to the methane reaction zone. According to the passive scalars introduced, it was shown that ignition occurs at mixtures rich in n-heptane vapour. Subsequently, consumption of both n-heptane and methane is rapidly increased and promoted by the high temperatures achieved. The competition of the two fuels makes autoignition retarded relative to the pure n-heptane case, but accelerated relative to the pure methane case.