In a first aspect of the present invention, there is provided a polymer composition comprising (i) one or more multistage polymer comprising (a) a first stage polymer having Tg of 0° C. or lower, (b) a subsequent stage polymer having Tg of 20° C. or higher, (ii) one or more phosphate salt of a multivalent cation, and (iii) one or more alkaline phosphate, in the amount of 100 ppm or more, measured as the weight of phosphorous, based on the dry weight of said multistage polymer. In a second aspect of the present invention, there is provided a mixture comprising the polymer composition of the first aspect described above and one or more matrix resin.In a third aspect of the present invention, there is provided a process for making a polymer composition comprising the steps of providing a multistage polymer latex, coagulating said multistage polymer latex, mixing said multistage polymer with an aqueous solution of an alkaline phosphate, and drying said multistage polymer to a water content of less than 1% by weight of water based on the dry weight of said multistage polymer, wherein said dried multistage polymer comprises 100 ppm or more, based on the dry weight of said multistage polymer, of phosphorous that is in the form of an alkaline phosphate.