A new class of interpolymer complexes - three-component interpolymer complexes with a low molecular mediator (IPCMs) -has been discovered. IPCMs consist of two identically charged polyelectrolytes and a low-molecular bifunctional base. The IPCM formed by polyacrylic acid, polyphosphate Na and various organic bases has been studied. It has been established that IPCMs represent equilibrium interpolymer compounds. It has been proved that a co-operative interaction of a low molecular mediator with each of the IPCM constituents is necessary for the existence of an interpolymer complex. A structure is proposed for IPCMs.