The theoretical analysis about the influence of high concentration and large concentration gradients on the hydrodynamics stability of the flow in the laminar boundary layer has been done. The results obtained with the linear stability theory show that the stability of flows increases when density depends on concentration. The decrease of concentration gradient leads to decrease of stability. In the cases when the increase of concentration leads : to increase (decrease) of viscosity, increase of stability can be observed, i.e. high concentrations lead to high (low) mass transfer rates in gases. Change in diffusivity does not influence stability. Additivity of the separated effects is observed.