Used to describe what happens when the flow rate of the mobile phase is so rapid that a true equilibrium between the phases cannot be reached Since neither the rate of transfer nor equilibrium is instantaneous, both the beginning and the end of the zone are broadened If the flow rate is decreased such that there is more time for equilibrium to be reached and if the stationary phase films are thin so that the solute molecules don't have to diffuse far to reach the stationary phase, mass transfer effects can be diminished