A nonlinear boundary-value problem describing the operation of an isothermal flow reactor with account for the substance flow relaxation is considered. The examined substance source is described by a kinetic law that corresponds to a definite type of autocatalytic reaction. Possible solutions under the Dankwerts standard boundary conditions are determined. The limiting form of equations of the relaxation model has been taken as a basis for the mathematical model. The change in the regimes of operation of reactors in the presence of three solutions of the process equations is analyzed.