The influence of the processing variables on the birefringence and polymer/gas interface distribution is analyzed for polystyrene moldings obtained by gas-assisted injection molding (GAIM) under various processing conditions. The processing variables studied were: melt and mold temperatures, shot size, gas pressure, injection speed, and gas-delay time. Measurements and viscoelastic simulations of the radial distribution of birefringence components, Delta n and n, n(pi) -n(theta theta), the variation of the average birefringence, (n(zz) -n(theta theta)), along the molding and polymer/gas interface along the length of spiral-shaped tubular moldings are presented. The polymer/gas interface distribution and flow stresses were simulated using a numerical scheme based on a hybrid finite element/finite difference/control volume method. The birefringence was calculated from the flow-induced stresses using the stress-optical rule. Simulations qualitatively agreed with measurements and correctly described the effect of the processing variables on the birefringence and the polymer/gas interface distribution in GAIM moldings. POLYM. ENG. SCL, 49:2350-2373, 2009. (C) 2009 Society of Plastics Engineers