Gel extraction is a new cyclic process of substances separation in solution according to their molecular size, which is based on phase transition in the swollen gel. In the article the equations of the process are obtained. The equations link the substance concentration in the concentrate and in the permeate, as well as the amount of these solutions, with the process parameters and the type of gel used in the process. It is assumed that all parameters of the process do not depend on the cycle number.