The ability to predict fluid behavior, such as velocity distribution or degree of mixing, is a critical step in designing industrial mixing processes. However, the majority of processing technologies are difficult to study using traditional approaches, due to the opacity/impermeability of the construction materials, as well as employed fluids, and geometric complexities of such systems. The current work applies a novel technique, positron emission particle tracking (PEPT), which allows for characterization of complex systems. PEPT relies on triangulation of gamma-rays emitted by a radioactive tracer particle, allowing the study of geometrically complex systems regardless of the system properties. This study compares the velocity distributions of a Newtonian fluid, glycerol, and a non-Newtonian fluid, guar gum solution (0.7%, w/w), flowing through 10 elements of a DN25 SMX mixer at 300 L/h. Axial velocity remained positive throughout, and no back-mixing was exhibited. The velocity components appeared to be independent of rheology, with the overall flow across 10 mixer elements resembling plug flow. Radial velocities were unimodally distributed around zero in the direction where no mixing was induced, while in the direction in which radial mixing is induced, the velocity distributions were either uni- or bimodal, depending on the geometry of the cross-section. (C) 2016 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers.