The present invention provides a dispersant for aqueous and non-aqueous systems which can disperse insoluble fine powders in non-aqueous liquids in a short period of time and give long-term dispersion stability to the resulting dispersions without allowing the formation of hard cake or precipitates. The dispersant is a compound of the formula ##STR00001## wherein SO is styrene oxide; R is a C.sub.8 C.sub.22 saturated or unsaturated group; x=1 100; y=1 3; z=1 100 and s=1 100. The dispersant of the invention is also particularly effective for dispersing additives into polymers such as polyesters, polyamides, polycarbonates, polyethylene and polypropylene.