Process for treating keratin fibers that have not been artificially dyed, in particular human keratin fibers such as hair, comprising: (i) a step consisting in the application to said fibers of a cosmetic composition comprising, in a medium cosmetically acceptable, one or more oxidized anionic polysaccharides represented by the following formula (I): P- (CHO) m (COOX) n (I) in which: P represents a polysaccharide chain selected from inulins, X is selected from a hydrogen atom, ions derived from an alkali metal or alkaline earth metal such as sodium or potassium, aqueous ammonia, organic amines such as monoethanolamine, diethanolamine, triethanolamine and 3-amino-1,2-propanediol and basic amino acids such as lysine , arginine, sarcosine, ornithine and citrulline, m + n is equal to or greater than 1, m is such that the degree of substitution of the polysaccharide with one or more aldehyde groups (DS (CHO)) is within the range of 0.001 a 2 and preferably 0.005 to 1.5, n is such that the degree of substitution of the polysaccharide with one or more carboxylic groups (DS (COOX)) is within the range of 0.001 to 2 and preferably 0.001 to 1.5 and (ii) a heating step of the keratin fibers, carried out at a temperature ranging from 25 ° C to 180 ° C.Proceso de tratamiento de fibras de queratina que no se han teñido artificialmente, en particular fibras de queratina humana como el cabello, que comprende: (i) un paso que consiste en la aplicación a dichas fibras de una composición cosmética que comprende, en un medio cosméticamente aceptable, uno o más polisacáridos aniónicos oxidados representados por la fórmula (I) siguiente: P-(CHO)m(COOX)n (I) en la que: P representa una cadena de polisacárido seleccionada a partir de inulinas, X se selecciona a partir de un átomo de hidrógeno, los iones derivados de un metal alcalino o un metal alcalinotérreo como sodio o potasio, amoniaco acuoso, aminas orgánicas como monoetanolamina, dietanolamina, trietanolamina y 3-amino-1,2-propanodio
