With monodispersed poly(methyl methacrylate/n-butyl acrylate/acrylic acid) [P(MMA/BA/AA)] seeded latex with a particle size of 485 nm and a solid content of 50 wt % as a medium, a series of stable P(MMA/BA/AA)/poly(styrene/acrylic acid) composite latexes with a high solid content (70 wt %) and low viscosities (500-1000 mPa(.)s when the shear rate was 21 s(-1)) was prepared in situ via simple two-step semicontinuous monomer adding technology. The coagulum ratio of polymerization was about 0.05 wt %. The particle size distribution of such latexes was bimodal, in which the large particle was about 589 nm and the small one was about 80 nm. The latexes combined good mechanical properties with good film-forming properties. Differential scanning calorimetry showed that the corresponding latex film had a two-phase structure. The morphology of the latex film was characterized with atomic force microscopy and scanning electron microscopy. (c) 2006 Wiley Periodicals, Inc.