Bubble-train flow consists of strings of bubbles separated by liquid slugs flowing cocurrently inside capillaries of circular or square cross section. Bubble-train flows have interesting properties and have been used to enhance heat and mass transfer rates inside capillaries. The quantitative analysis of heat and mass transfer processes related to this flow requires a detailed knowledge of flow patterns inside the liquid slugs. High-speed video imaging was used to characterize liquid flow patterns. Velocity distributions inside liquid slugs were determined by particle imaging velocimetry (PIV). Video images recorded in a reference frame moving with the bubbles indicate the presence of recirculating patterns with a high degree of mixing. Depending on the capillary number of the flow, counter rotating vortices or a complete bypass flow inside the liquid slug were observed. The position and size of these vortices and the magnitude of the liquid velocity can be accurately predicted by theory. Mixing inside the liquid slug can be characterized by the average recirculating time of a particle of fluid trapped in the liquid vortex.