Continuous gas sparging inside urtrafiltration hollow fibres (with the feed) has been shown to be effective in enhancing permeate flux by reducing particle deposit on the membrane wall. The present study focuses on the hydrodynamic characterisation of gas-liquid two-phase flow inside glass capillaries in order to understand the mechanisms involved in the process improvement. In capillaries of inner diameter less than 4 x 10(-3) m, experiments showed that the how consisted of a slug how: a succession of gas and liquid slugs rising up the pipe. As very few data are available concerning this type of flow, it was visualised inside the glass capillaries so that its characteristics could be observed. Gas slug velocity, which is different from the mean velocity of the flow, was calculated and a linear relationship found between the two velocities. Gas and liquid slug lengths and frequency were determined and plotted Versus operating parameters such as gas and liquid superficial velocities, capillary inner diameters and fluid properties.