The present invention concerns a process for preparing a polymer comprising at least one gel polymerization step (E) in which the following are contacted: ethylenically unsaturated water-soluble monomers, the same or different; a source of free radicals adapted to the polymerization of the said monomers, typically a redox system; and a radical polymerization control agent, preferably comprising a thiocarbonylthio —S(C═S)— group; with a concentration of monomers in the reaction medium of step (E) sufficiently high to cause gelling of the medium if polymerization was conducted without the control agent.