Mixing layers are a fundamental phenomenon that occurs in many more complex flows such as jets, counter-flows and recirculating flows. The importance of mixing layers as a building-block in fluid mechanics is evident in the large number of computational, experimental and theoretical studies devoted to the topic. With its simple configuration and easy control of flow parameters, mixing layers were one of the first flows that became amenable to direct numerical simulation (DNS). Examples of DNS of incompressible (Moser and Rogers,
) and compressible (Vreman et al.,
) can be found. However, these studies and most others have considered only single-species (usually air) mixing layers, in which the two streams have different flow properties but the same fluid properties. Relatively few studies by DNS have considered binary-species (Okong’o et al.,
) and multi-species (Knaus and Pantano,
; Echekki and Chen,
; Zheng et al.,
))mixing layers in which the fluid properties of different streams differ. Such binary- and multi-species mixing layers are of special importance in the process and energy (i.e. combustion) industry.