This research prepared polyelectrolyte complex particles of chitosan-poly(acrylic acid) (CS-PAA) through polymerization of acrylic acid (AA) in the presence of chitosan (CS). The prepared CS-PAA complex particles had positive zeta potential and dispersed very well in the aqueous solution. Structure and morphology of complex particles were investigated with the changes in the feeding molar ratio of glucosamine unit in CS to AA monomer. It was found at a molar ratio of 1.0/1.1, a hollow structure in complex particles was observed after polymerization. From solid state C-13-NMR and FTIR analyses, the results confirmed that CS-co-PAA copolymer was also produced during polymerization in addition to the PAA homopolymer, and both constituted in the structure of complex particles. The synthesized complex particles were environmentally sensitive in which their mean diameter could change with the pH value of medium. Moreover, the complex particles showed a continuous release of the encapsulated doxycycline hyclate up to 8 days. These complex particles with environmentally sensitive properties are expected to be utilized in the hydrophilic drug delivery system. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1659-1670, 2011