X-ray photoelectron spectroscopy is used to study the surface segregation of siloxane in blends of poly(vinyl chloride) with siloxane-urethane-ethyleneoxide oligomers. At high concentrations of the oligomeric additive, the surface segregation of siloxane in the blends strongly depends on the molecular architecture of the additive : the segregation is much higher when the siloxane blocks are at the ends of the oligomeric chains. At low additive concentrations, the surface segregation of siloxane is governed solely by the siloxane concentration in the bulk.