Fe-ZSM-11 zeolites, prepared by novel sol-gel process with Fe²⁺ and/or Fe³⁺ as active sites incorporated by reproducible post-synthesis methods were obtained. Recently we reported the relationship between the catalytic activity and the nature of the active sites [1]. The experiment design-response surface methodology is used in this work to optimize the selective reduction of NO_x. The main objective is to define the response surfaces, finding the best combination of different parameters in order to optimize the process, for this reason, the experiments design methodology was applied reducing the operation costs, achieving efficiency and effectiveness in the process. The application of this methodology leads us to a better understanding of the influence of the different factors such as: number and strength of the acid sites, Fe²⁺/Fe³⁺ relation, [NO₃*] and [NO₂*] adsorbed intermediates, the concerted reaction mechanism proposed in the previous work, and the effect of the reaction temperature and their interactions. The statistic model applied in this work is a powerful tool to interpret the overall process from the multivariate parameters. According to the statistic methodology applied, the best operation conditions and catalysts characteristics can be identified, allowing us to  re-design the catalyst to enhance the NOx conversion and N₂ and CO₂ selectivity at optimum reaction temperature.