The invention relates to a method for the controlled oxidation of oligo- and/or polysaccharides containing free --CH.sub.2OH primary hydroxyl groups, using an oxidising agent. The invention is characterised in that the oxidation reaction is performed in a densified fluid which is inert in relation to the oxidising agents employed. The invention is suitable for use, for example, in the food, paint, paper, textile, agricultural and pharmaceutical industries, in the medical, biomedical or paramedical fields, in the surgical field, in the cosmetic industry and as agents for the complexing or sequestering of metal ions, heavy metals and radioactive elements in the nuclear industry.