Acid polysaccharides characterised by the concomitant presence of partial esters with non-polysaccharide carboxylic acids and esters between the acid groups of the initial polysaccharide and the alcohol groups of the repetitive units, with the formation of crosslinking among the polysaccharide chains. Process for the preparation of these derivatives comprising the reaction, in homogenous phase in the protic, polar solvent formamide, of the salt of a monovalent inorganic cation of the selected carboxylated polysaccharide with an anhydride of an alkylcarboxylic acid in the presence of a basic catalyst containing an atom of trisubstituted nitrogen. The formamide hydrolysis leads to the formation of a formate ester.
