Abstract A rigid PVC formulation was optimized for processing window and cost using sequential simplex techniques in combination with desirability functions. This optimization was performed on seven of the ten ingredients from a ‘standard’ siding compound with a relatively limited number of experiments. The processing window contour mapping technique was used to evaluate the effects of formulation changes. This was combined with the formulation cost by using desirability functions to give an overall response for the simplex to optimize. The basic mechanics of sequential simplex and desirability functions are described along with the results of the optimization.