We successfully developed a novel and simple method for preparation of monodisperse thereto-sensitive poly(N-isopropylacrylamide) (PNIPAM) hollow microcapsules at the interface of water-in-oil (W/O) single emulsions at a temperature below the lower critical solution temperature (LCST) of PNIPAM. The prepared PNIPAM microcapsules are featured with hollow structures and thin membranes, high monodispersity, excellent reversible thereto-sensitivity and fast response to environmental temperature. This approach exhibits great interests in preparing monodisperse thereto-sensitive microcapsules for encapsulating bioactive materials or drugs requiring mild encapsulation conditions, because of the flexibility in choosing substances being dissolved in the aqueous phase. The preparation methodology demonstrated in this study provides a unique approach for preparing monodisperse hollow polymeric microcapsules with W/O single emulsions. (c) 2007 Elsevier Inc. All rights reserved.