The present invention relates to a water-redispersible powder based on film-forming polymers which are substantially water-insoluble and which are prepared by aqueous emulsion polymerization of monomers containing ethylenic unsaturation with a "core/shell" structure comprising a hydrophobic core based on a polymer exhibiting a Tg (glass transition temperature) of between -30.degree. and +25.degree. C., preferably between -10.degree. and +20.degree. C., and a hydrophilic shell based on a polymer exhibiting a Tg higher than +55.degree. C., preferably higher than +90.degree. C., characterized in that the shell is bonded to the core by covalent bonds resulting from the reaction of an agent for grafting the shell onto the ethylenic unsaturations remaining in the core, and is substantially insoluble in an alkaline medium.