Static mixers are widely used in continuous mixing, heat and mass transfer processes and chemical reactions. However, a proper understanding of the flow pattern is still missing when theologically complex fluids are involved. This paper presents a study of the Residence Time Distribution (RTD) determination in a Sulzer SMX static mixer. A two-parameter RTD model, based on each mixing element, was proposed to describe the flow pattern of theologically complex fluids in a such mixer. It was shown that in this model the flowrate fraction across the plug flow component increases with the generalized Reynolds number. However, the volumetric fraction of the plug flow component depends not only on the generalized Reynolds number, but also on the number of mixing elements, apparently due to memory effects of viscoelastic fluids.