This paper describes an experimental investigation of the density field of coaxial jets with large density differences. These flows are characterised by a lon velocity high density inner jet surrounded by a high velocity-low density annular jet. The density field was determined by a thermo-anemometric method based on a new version of an aspirating probe. Measurements show that mixing is directly dependent upon the flow dynamics in the near-field region. As a result, density effects on mixing as well as on the flow dynamics are rather well taken into account by considering the specific outer to inner jet momentum flux ratio nl and not separately the density and the velocity ratios. Fur a gic en value of M however, a slight enhancement of mixing is found for density ratios much smaller than one (approximate to0.14).