The effects of freestream density ratio on the mixing and combustion in a high Reynolds number, subsonic, gas-phase, non-buoyant, two-dimensional turbulent mixing layer, have been investigated. Measurements of temperature rise (heat release) have been made which enable us to examine the effect of freestream density ratio on several aspects of the mixed fluid State within the turbulent combustion region. In experiments with very high and very low stoichiometric mixture ratios ("flip" experiments), the heat release from an exothermic reaction serves as a quantitative label for the lean reactant freestream fluid that becomes molecularly mixed. Properly normalized, the sum of the mean temperature rise profiles of the two flip experiments represent the probability of fluid molecularly mixed at any composition. The mole fraction distribution and number density profile of the mixed fluid can also be inferred from such measurements. Although the density ratio in these experiments was varied by a factor of thirty, profiles of these quantities show little variation, with integrals varying by less than 10%. This insensitivity differs from that of the composition of molecularly mixed fluid, which is very sensitive to the density ratio. While the profiles of composition exhibit some similarity of shape, the average composition of mixed fluid in the layer varies from nearly 1:2 to over 2:1 as the density ratio is increased. A comparison of data and available theory for this offset or average composition is discussed.