Latex blend films were prepared from mixtures of two types of particles in dispersion, one composed of a high-T-g polymer [poly(methyl methacrylate), PMMA]; the other a copolymer of butyl methacrylate and butyl acrylate [P(BMA-co-BA)] with T-g less than or equal to 10 degrees C. Transparent films were obtained under air-drying conditions if the PMMA particles had diameters less than similar to 250 nm and if the volume fraction of low-T-g latex polymer exceeded a certain critical fraction Phi(c). Values of Phi(c) varied over a narrow range (0.40-0.50) with P(BMA-co-BA) particle size, and were independent of the T-g of the soft latex, ranging from ca. -35 to +10 degrees C. Film morphologies were examined by scanning electron microscopy and by freeze-fracture transmission electron microscopy. In all films, the hard particles retain their original size and spherical shape. In the transparent films, they are uniformly distributed in a polymer matrix generated from deformed soft particles, whereas clustering of PMMA microspheres is observed in turbid films. Various factors, such as increasing the size ratio between the two types of particles, removing of surfactant in the systems, and annealing of the films after drying, disrupt the uniform particle packing required for transparent films.