Composite latex of P(St-co-BA)/P(MMA-co-BA) was prepared by two-step semi-continuous emulsion polymerization under starved conditions Optical properties, water resistance and heat aging process of the film formed by the composite latex were examined. It was revealed that annealing temperature played an important role on the structure of the composite latex film The surface of the film prepared at room temperature was very rough Many micro-cracks were observed both on the surface and inside of the film. Increase of annealing temperature assisted film formation. There was a critical temperature at which the film properties, such as water resistance and transparency, were significantly Improved However, it was worth to note that phase rearrangement had occurred when the annealing temperature was higher than critical temperature. Micro-pinholes and micro-cracks were detected on the surface of the film Meanwhile, the properties of the film, such as transparency and water adsorption, became remarkable deteriorated (C) 2010 Elsevier Inc All rights reserved