A new formulation of the multicomponent boundary‐layer problem which is convenient for numerical calculations is developed by a modification of the usual Chapman‐Enskog procedure. The complete set of boundary‐layer equations turn out to be a set of first‐order differential equations and a set of algebraic equations which are linear in the fluxes. The method is extended to include chemically reacting mixtures as well as nonreacting polyatomic molecules. It is shown that the method can be extended to two‐temperature fluids when one of the species has a small mass and to mixtures where it is necessary to retain higher‐order terms in the polynomial expansions used in the Chapman‐Enskog procedure.