To stabilize vitamin A in a cosmetic/dermatological formulation, we present here a new encapsulation method based on polymer microspheres having a localized "proton-buffering" capacity. Poly(methyl methacrylate)-g-polyethylenimine (PNMA-g-PEI) was prepared by direct condensation grafting of PEI onto poly(methyl methacrylate-co-methyl acrylic acid). The reaction was confirmed by FT-IR analysis showing the amide vibration at 1,550 cm(-1). Elemental analysis indicated that the weight content of the grafted PEI was 1.6% (w/w). Vitamin A was encapsulated into PMMA-g-PEI microspheres by using an oil-in-water (O/W) single emulsion method. The presence of PEI moiety dramatically improved the chemical stability of vitamin A in microspheres. Vitamin A encapsulated within PMMA-g-PEI microspheres maintained 91% of its initial activity after 30-day incubation at 40degreesC, while only maintaining 60% within plain PMMA microspheres. This study demonstrates that proton-buffering within hydrophobic polymer matrix is a useful strategy for stabilizing "acid-labile" active ingredients. (C) 2004 Wiley Periodicals, Inc.