We study an electron-hole system in double quantum wells theoretically using the powerful Green's function technique. We demonstrate that there is a temperature interval over which an abrupt jump in the value of the ionization degree occurs with an increase of the carrier density or temperature. The opposite effect, the collapse of the ionized electron-hole plasma into an insulating exciton system, should occur at lower densities. In addition, we predict that under certain conditions there will be a sharp decrease of the ionization degree with increasing temperature-the anomalous Mott transition. We discuss how these effects could be observed experimentally.