An exact solution is obtained for the suction flow problem. Hydrodynamical theory is shown to be valid only under a certain limit. The neighborhood of the boundary and the neighborhood of infinity are shown to always be kinetic layers. The exact solution is then used for a general determination of slip coefficients to be used as perturbation boundary conditions in the Chapman‐Enskog‐Hilbert method. One explicit result of this, is the resolution of the problem of slip velocity in a pressure gradient field.