Abstract: A process for preparing a cationic polymer which comprises polymerizing a monomer mixture containing acrylamide and a cationic alkenyl monomer represented by the general formula: ##STR1## wherein R.sup.1 represents H or CH.sub.3 ; R.sup.2 and R.sup.3 represent independently CH.sub.3 or C.sub.2 H.sub.5 ; R.sup.4 represents H, CH.sub.3, C.sub.2 H.sub.5 or a benzyl group; A represents --OC.sub.n H.sub.2n -- (wherein n=2 or 3), ##STR2## or --NC.sub.n H.sub.2n -- (wherein n=2 or 3); and X.sup.- represents Cl.sup.-, Br.sup.-, I.sup.-, CH.sub.3 SO.sub.4.sup.-, C.sub.2 H.sub.5 SO.sub.4.sup.-, 1/2SO.sub.4.sup.-- or CH.sub.3 COO.sup.-, in the form of a sheet in an aqueous medium, characterized in that the composition ratio of acrylamide/cationic alkenyl monomer is within the range of 95/5 to 5/95, and the polymerization is carried out in the presence of a phosphite. The polymers obtained according to this process are free from insolubles produced under the influence of oxygen.