Experimental design was used to optimize the processing parameters for the decomposition of poly (vinyl chloride). Factorial design and face centered composite design (FCC) were applied to determine the optimum conditions. A total of 10 g PVC powder was mixed with different amounts of zinc stearate (ZnSt(2)) and natural zeolite and tested for thermal stability. Factorial fitted model was explained by first order pattern due to the significant main effect regression constants, and FCC model was described by second order model owing to higher order polynomial coefficients. FCC design was superior to factorial design as FCC considers not only its pure quadratic effects contribution but also its higher overall desirability for thermal stability of PVC. For factorial design the optimum conditions were determined as 163.06 mg for ZnSt(2), 399.99 mg for zeolite and 140 degrees C for temperature with desirability of 0.933. However, 400 mg for ZnSt(2), 333.24 mg for zeolite, and 140 degrees C for temperature with desirability of 0.956 were obtained as the optimum conditions by FCC design. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 1811-1822, 2010