A conceptual approach to mass transfer at the interface between a liquid and a gas. The gas–liquid interface is assumed not to be at steady state but is continually being refreshed by liquid from the bulk below. A theory assumes that an element of liquid reaches the interface for a fixed time depending on the hydrodynamic conditions and arrives with a particular concentration. The residence time is assumed to be short enough for the transferred component not to penetrate the element far enough to affect the bulk concentration. It was proposed by R. Higbie in 1935.