In the polymers industry, models are heavily parametrizised, and the effect of each parameter on model outputs has not been extensively studied. A wide range of values for most of model’s parameters can be found in literature [1], a thorough analysis regarding the model’s sensitivity to the parameters’ values is needed to find the set of parameters that have the most impact in the output results and consequently deserve an extra effort and care during theirestimation. In this work, a global sensibility analysis of a styrene emulsionpolymerization reactor model is carried out in order to determine the set of critical parameters.