Abstract: Mass exchange characteristics of cavities for scramjet applications under supersonic cold and combustion flow were investigated.The hybrid RANS/LES(Reynolds-averaged Navier-Stokes/large-eddy simulation) approach was used to simulate numerically unsteady state flowfields.The influences of the cavity parameters and the combustion on the mass exchange characteristics were investigated systemically.The mass exchange process in the reactive flow was described for the first time by using atom-tracking method.The hybrid RANS/LES data suggest that longer cavities have a longer residence time both in cold and combustion flow.The aft wall angle has a significant influence on the residence time just in the cold flow.For the same cavity,residence time reduces obviously in the combustion flow as compared with that in the cold flow.