Based on nonequilibrium thermodynamics we formulate a general theory of the kinetics of adsorption, desorption, and dissociation of gases at surfaces. We begin with a concise formulation of dissociation equilibrium at surfaces and then derive the kinetic equations for adsorption, desorption, and dissociation. For the explicit calculations we employ a lattice gas model for homonuclear molecules with both atoms and molecules present on the surface. Lateral interactions between all species are accounted for. In a series of model calculations we discuss equilibrium properties, such as heats of adsorption, and examine the role of dissociation dis-equilibrium on the time evolution of an adsorbate during temperature programmed desorption. The further effect of (attractive or repulsive) lateral interactions on the kinetics is elucidated in further model calculations. As a realistic example we apply the theory to oxygen on Pt(111).