We present experimental measurements of concentration fields within a hemodialyzer usings X-ray computed tomography (CT). The measured concentrations represent a volume average value for the shell and lumen fluids. Although average values for a cross-section decrease uniformly from lumen inlet to outlet large variations in concentration occur within a cross-section. We attribute these concentration variations to variations in shell flow; regions of lower shell flow have higher concentrations. The regions of lower shell flow do not always correspond to regions of higher fiber packing. Distribution of fluid within the fiber bundle also is important.