Most of the bio-reactive processes now in development by Pharmaceutical and Biotechnological firms will be based on the culture of shear-sensitive mammalian. insect or plant cells. To properly address the need of designing, optimizing, and scaling up such bioprocesses, a precise characterization of the flow and mixing behavior of typical and novel bioreactor geometries is required. Using experimental visualization techniques (3-D UV light pattern visualization, 2-D laser-induced fluorescence techniques, and acid base reactions in the presence of pH indicators) we experimentally describe and analyze common mixing pathologies occurring in frequently used bioreactor geometries: heterogeneity, coexistence of regular and chaotic regions, flow segregation, compartmentalization, flow bypass, cell focusing, etc. We also analyze the role of baffles on mixing in bioreactors operated at low and moderate Re. and discuss the risk and validity of assuming perfectly mixed conditions in a bio-reaction system. (c) 2005 Elsevier Ltd. All rights reserved.