The deformation mechanism of a styrene/n-butyl acrylate copolymer latex film subjected to uniaxial tensile stress was studied by small-angle X-ray scattering. The influence of annealing at 23, 60, 80, and 100 degrees C for 4 h on microscopic deformation processes was elucidated. It was demonstrated that the microscopic deformation mechanism of the latex films transformed gradually from nonaffine deformation behavior to affine deformation behavior with increasing annealing temperature. This behavior was attributed to the interdiffusion of polymeric chains between adjacent latex particles in the film, the extent of which increases with annealing temperature. The polycrystalline structure of the latex films was preserved even after annealing for 4 h at temperatures 80 K above the glass transition temperature; i.e., the structure is not completely homogenized by interdiffusion.