The stability and receptivity of high speed boundary layers in binary mixtures of oxygen are investigated including chemical non-equilibrium effects. The analysis is conducted for two-dimensional perturbations for both an inviscid and a viscous model and the results are compared to examine the impact of viscous effects. It is found that the viscosity effects stabilize the flow, and the main impact on temperature and mass fraction perturbations occurs in the viscous sublayer and in the critical layer. The biorthogonal eigenfunction system is used to study the receptivity of the boundary layers including real gas effects. The results demonstrate that the receptivity coeffcients have two maxima associated with the branching points in the discrete spectrum. The maxima become stronger when the wall temperature is low.