In this work, poly(N-isopropylacrylamide-co-acrylic acid) (poly(NIPAAm-AA)) copolymer latex particles (microgels) were synthesized by the method of soapless emulsion polymerization. Poly(NIPAAm-AA) copolymer microgels have the property of being thermosensitive. The concentration of acrylic acid (AA) and crosslinking agent N,N'-methylenebisacrylamide were important factors to influence the lower critical solution temperature (LCST) of poly(NIPAAm-AA) microgels. The effects of AA and crosslinking agent on the swelling behavior of poly(NIPAAm-AA) microgels were also studied. The poly(NIPAAm-AA) copolymer microgels were then used as a thermosensitive drug carrier to load caffeine. The effects of concentration of AA and crosslinking agent on the control release of caffeine were investigated. How the AA content and crosslinking agent influenced the morphology and LCST of the microgels was discussed in detail. The relationship of morphology, swelling, and control release behavior of these thermosensitive microgels was established. A new scheme was proposed to interpret the control release of the microgels with different morphological structures. (c) 2008 Wiley Periodicals, Inc.