The boundary layer approximation is applied to the equations governing the mixing zone between two parallel semi-infinite streams. The two streams differ in composition and temperature, but they are not necessarily in chemical equilibrium. Closed form solutions are obtained for the approach to equilibrium in the mixing zone in the case of equal velocitiesof the two streams. Frozen flow and equilibrium flow appear as limiting cases of the general solution. A linearized reaction rate law is used for the no-equilibrium stituations. These solutions indicate that the major factor contributing to non-equilibrium conditions outside of the mixing zone is the presence of non-equilibrium conditions outside of the mixing zone. An approximate solution to the case of unequal free stream velocities is obtained using an equilibrium condition found to be applicable in the constant velocity case. The results of these studies are used to provide a qualitative indication of the effects of dissociation and ionization reactions on the flow field immediately behind a hypersonic body. It is found that the presence of reactions in various regions of this flow field causes individual and discernible changes in the wake expansion angle and in the base pressure.