A kinetic model describing physisorption and chemisorption of gas particles on a crystal surface is introduced. A single kinetic equation is used to model gas and physisorbed particles interacting with an average potential and colliding with phonons. This equation is coupled to a kinetic equation describing localized chemisorbed species. A modified kinetic entropy is introduced for the coupled system and the H theorem is established. Using a fluid scaling and the Chapman-Enskog asymptotic method, fluid boundary conditions for the physisorbed and chemisorbed species are derived from the kinetic model.