Process for preparing a composition comprising: (i) alkyl(alkyl guloside)uronates of formulae (Ia), (Ib) and (Ic): or (ii) a mixture of alkyl(alkyl guloside)uronates of formulae (Ia), (Ib) and (Ic) and of alkyl(alkyl mannoside)uronates of formulae (IIa), (IIb) and (IIc): wherein - R 1 is a linear or branched, saturated or unsaturated alkyl chain having from 2 to 22 carbon atoms; - R 2 is a hydrogen atom, R 1, an alkali metal atom, an alkaline-earth metal atom, or a quaternary ammonium group of formula: - wherein each of R 3 to R 6 is independently a hydrogen atom, an alkyl having from 1 to 6 carbon atoms or a hydroxyalkyl having from 1 to 6 carbon atoms, and characterized in that said process comprises: a) a step of hydrolysis of poly(oligo)guluronates, of oligoalginates, of alginates; b) a step of esterification and glycosylation of the hydrolysate resulting from step a) with a linear or branched, saturated or unsaturated alcohol of formula ROH, having from 1 to 4 carbon atoms; c) a step of trans-esterification and trans-glycosylation of the reaction medium resulting from step b) with a linear or branched, saturated or unsaturated alcohol of formula R'OH having from 2 to 22 carbon atoms; d) optionally a step of neutralization of the reaction medium resulting from step c) in the presence of water and of a base M x in which M is an alkali metal or alkaline-earth metal, and x is the valency. (i) alkyl(alkyl guloside)uronates of formulae (Ia), (Ib) and (Ic): or (ii) a mixture of alkyl(alkyl guloside)uronates of formulae (Ia), (Ib) and (Ic) and of alkyl(alkyl mannoside)uronates of formulae (IIa), (IIb) and (IIc): wherein - R 1 is a linear or branched, saturated or unsaturated alkyl chain having from 2 to 22 carbon atoms; - R 2 is a hydrogen atom, R 1, an alkali metal atom, an alkaline-earth metal atom, or a quaternary ammonium group of formula: - wherein each of R 3 to R 6 is independently a hydrogen atom, an alkyl having from 1 to 6 carbon atoms or a hydroxyalkyl having from 1 to 6 carbon atoms, and characterized in that said process comprises: a) a step of hydrolysis of poly(oligo)guluronates, of oligoalginates, of alginates; b) a step of esterification and glycosylation of the hydrolysate resulting from step a) with a linear or branched, saturated or unsaturated alcohol of formula ROH, having from 1 to 4 carbon atoms; c) a step of trans-esterification and trans-glycosylation of the reaction medium resulting from step b) with a linear or branched, saturated or unsaturated alcohol of formula R'OH having from 2 to 22 carbon atoms; d) optionally a step of neutralization of the reaction medium resulting from step c) in the presence of water and of a base M x in which M is an alkali metal or alkaline-earth metal, and x is the valency. - R 1 is a linear or branched, saturated or unsaturated alkyl chain having from 2 to 22 carbon atoms; - R 2 is a hydrogen atom, R 1, an alkali metal atom, an alkaline-earth metal atom, or a quaternary ammonium group of formula: - wherein each of R 3 to R 6 is independently a hydrogen atom, an alkyl having from 1 to 6 carbon atoms or a hydroxyalkyl having from 1 to 6 carbon atoms, and characterized in that said process comprises: a) a step of hydrolysis of poly(oligo)guluronates, of oligoalginates, of alginates; b) a step of esterification and glycosylation of the hydrolysate resulting from step a) with a linear or branched, saturated or unsaturated alcohol of formula ROH, having from 1 to 4 carbon atoms; c) a step of trans-esterification and trans-glycosylation of the reaction medium resulting from step b) with a linear or branched, saturated or unsaturated alcohol of formula R'OH having from 2 to 22 carbon atoms; d) optionally a step of neutralization of the reaction medium resulting from step c) in the presence of water and of a base M x in which M is an alkali metal or alkaline-earth metal, and x is the valency. a) a step of hydrolysis of poly(oligo)guluronates, of oligoalginates, of alginates; b) a step of esterification and glycosylation of the hydrolysate resulting from step a) with a linear or branched, saturated or unsaturated alcohol of formula ROH, having from 1 to 4 carbon atoms; c) a step of trans-esterification and trans-glycosylation of the reaction medium resulting from step b) with a linear or branched, saturated or unsaturated alcohol of formula R'OH having from 2 to 22 carbon atoms; d) optionally a step of neutralization of the reaction medium resulting from step c) in the presence of water and of a base M x in which M is an alkali metal or alkaline-earth metal, and x is the valency.