Process for the preparation of an aqueous polymer solution containing at least one cationic or amphoteric (co)polymer, comprising the following steps: - so-called Hofmann degradation reaction, in aqueous solution, in the presence of an earth alkaline and/or alkaline hydroxide and an earth alkaline and/or alkaline hypohalogenure on a (co)polymer base of at least one non-ionic monomer chosen from the group including acrylamide, methacrylamide, N, N-dimethylacrylamide, and acrylonitrile; 1/ obtaining of an aqueous polymer solution of viscosity V1 containing at least one cationic or amphoteric (co)polymer derived from the Hofmann degradation reaction; 2/ post-processing of this aqueous polymer solution of viscosity V1 from step 1/; characterized in that post processing of step 2/ comprises the following successive steps: (a) (a1) measurement of the V1 viscosity of aqueous polymer solution from step 1/; then (a2) adjustment of the pH of the aqueous polymer solution from step (a1) to a value greater than or equal to 5.5, said adjustment affording an increase of V1 viscosity of the aqueous polymer solution up to a V2 value corresponding to at least 1.5 times the V1 value while being less than 100,000 mPa.s; viscosities increase without adding a polyfunctional compound; viscosities are measured with a Brookfield viscometer, at 25°C with an LV module; (b) lowering of the pH of the aqueous polymer solution from step (a2) to a value between 2 and 5. - so-called Hofmann degradation reaction, in aqueous solution, in the presence of an earth alkaline and/or alkaline hydroxide and an earth alkaline and/or alkaline hypohalogenure on a (co)polymer base of at least one non-ionic monomer chosen from the group including acrylamide, methacrylamide, N, N-dimethylacrylamide, and acrylonitrile; 1/ obtaining of an aqueous polymer solution of viscosity V1 containing at least one cationic or amphoteric (co)polymer derived from the Hofmann degradation reaction; 2/ post-processing of this aqueous polymer solution of viscosity V1 from step 1/; 1/ obtaining of an aqueous polymer solution of viscosity V1 containing at least one cationic or amphoteric (co)polymer derived from the Hofmann degradation reaction; 2/ post-processing of this aqueous polymer solution of viscosity V1 from step 1/; (a) (a1) measurement of the V1 viscosity of aqueous polymer solution from step 1/; then (a2) adjustment of the pH of the aqueous polymer solution from step (a1) to a value greater than or equal to 5.5, said adjustment affording an increase of V1 viscosity of the aqueous polymer solution up to a V2 value corresponding to at least 1.5 times the V1 value while being less than 100,000 mPa.s; viscosities increase without adding a polyfunctional compound; viscosities are measured with a Brookfield viscometer, at 25°C with an LV module; (a1) measurement of the V1 viscosity of aqueous polymer solution from step 1/; then (a2) adjustment of the pH of the aqueous polymer solution from step (a1) to a value greater than or equal to 5.5, said adjustment affording an increase of V1 viscosity of the aqueous polymer solution up to a V2 value corresponding to at least 1.5 times the V1 value while being less than 100,000 mPa.s; viscosities increase without adding a polyfunctional compound; viscosities are measured with a Brookfield viscometer, at 25°C with an LV module; viscosities increase without adding a polyfunctional compound; viscosities are measured with a Brookfield viscometer, at 25°C with an LV module; (b) lowering of the pH of the aqueous polymer solution from step (a2) to a value between 2 and 5. <EOS>