The movement of gas species across the air–water interface is a central aspect of biogeochemical cycling and plays a critical role in controlling not only the composition of the atmosphere, but the chemistry of aquatic and marine systems. This chapter shows how kinetic rate laws describing the transfer of gas species into and out of aqueous solution can be integrated into multicomponent chemical reaction models and shows a fully worked calculation, using carbon dioxide efflux from a biologically active lake as an example.