The addition of a small amount of a poor solvent impurity (methanol) to a theta solvent (cyclohexane) is found to cause appreciable swelling (approximate to30% increase of the average brush height) in a model end-grafted polystyrene (PS) brush layer. This unusual type of swelling is not observed if octadecyltrichlorosilane (OTS) is first grafted to the portion of the silicon substrate uncovered by the grafting end-groups of the PS chains. Brush swelling in the absence of OTS surface protection is interpreted as arising from a segregation of methanol to the solid substrate and the resulting modification of the polymer-surface interaction. We also observe that the addition of a small amount of methanol to an adsorbed PS layer exposed to cyclohexane causes rapid film delamination from the silicon substrate. Together these observations imply a strong influence of surface active impurities on the structure and adhesive stability of polymer layers. (C) 2004 Wiley Periodicals, Inc.