Concentration field-flow fractionation (CFFF) is proposed as a possible new subtechnique of field-flow fractionation (FFF). The equations for solute retention in CFFF are obtained as a function of solute distribution and the solute chemical potential increment, δμ°c, across the flow channel. The minimum practical value of δμ°cis related to the necessary increment, δCc, in the concentration of one component of a mixed solvent across the column. This leads to criteria for δCc, for the solvent component's flux, and for the channel width.