A process for the simultaneous substitution and crosslinking of a polysaccharide via its hydroxyl functional groups, in an aqueous phase, which includes the following steps: a polysaccharide is placed in an aqueous medium, it is brought into the presence of at least one precursor of a substituent, it is brought into the presence of a crosslinking agent, the substituted and crosslinked polysaccharide is obtained and isolated, wherein process is carried out in the presence of a basic or acidic catalyst, the concentration of which is between 3.16×10−7 and 0.32 mol/L, and at a temperature of less than 60° C. In one embodiment, the polysaccharide is in the form of a gel or hydrogel which is used in particular as augmentation biomaterial.