This paper reports non-invasive observations of flow characteristics inside a hollow fiber membrane lumen under suction conditions using X-ray microimaging (XMI) techniques. The observation revealed that for hydrophobic polypropylene (PP) hollow fiber membranes, bubbles, ranging in size from about 50 to 200 mu m existed in some local regions of the fiber lumen. An in situ suction pressure monitor revealed that the presence of the bubbles inside the fiber lumen caused considerable fluctuations in the transmembrane pressure during the filtration. The XMl observation identified two types of bubbles, stagnant and mobile. The stagnant bubbles, whose shape changed with increasing suction pressure, eventually reached a stable geometry of a semi-sphere with a flat bottom. These bubbles were observed to stick firmly to some local regions of the fiber lumen. Contact angle analysis shows that the existence of the stagnant bubble could be related to the hydrophobicity of the local region. The mobile bubbles were observed to grow and detach from the wall under suction conditions, indicating a possible role of cavitation in the filtration with hollow fiber membranes. A crevice model was used to explain the formation of the nuclei for cavitation on the fiber internal surface. In addition, this study also confirmed that X-ray microimaging is a useful tool for non-invasive observation of two-phase flow phenomenon inside fiber lumens. (c) 2007 Elsevier B.V. All rights reserved.