Blend emulsion of 2 kinds of particles, in which composite polymer particles were prepared by utilizing the stepwise heterocoagulation method proposed by the authors in 1990, was cast to prepare a microheterogeneous film. In the film, a discontinuous phase consisted of large anionic hard polymer particles and a continuous one consisted of small cationic soft polymer particles with the content of 30 wt % as film-forming additives. The storage stabilities of the blend emulsions after the stepwise heterocoagulation were examined under various conditions. The morphology of the film prepared was estimated from attenuated total reflectance Fourier transform infrared spectroscopic and dynamic mechanical spectroscopic measurements.