A linear mathematical model for gas—liquid chromatography is presented for application to either analytical or larger-scale equipment. The model includes the effects of axial dispersion, interparticle mass transfer, intraparticle gas diffusion, and diffusion in a thin, uniform thickness liquid film. The governing differential equations are solved in the Laplace transform domain for a general input pulse. Temporal moments of the output pulses are expressed in terms of geometrical, equilibrium and transport parameters. The moment theory is related to HETP and to resolution of separation. The optimization of the process and scale-up to industrial size are discussed.