A so-called "slug-flow" capillary microreactor has been proposed for the investigation of mass-transfer-limited liquid-liquid reactions. Internal circulation within the slug leads to an intensified and tunable mass transfer. Understanding the development of the circulatory flows and the influence of operating parameters upon them is thus crucial. In this study, experiments were carried out to visualize the internal circulations using particle image velocimetry (PIV) technique. State-of-the-art computational fluid dynamics (CFD) simulations were used to predict the internal circulation within the liquid slugs and a CFD particle tracing algorithm employed to visualize them. Each slug was modeled as a distinct single-phase flow domain. The effect of the flow velocity and slug length on the velocity profile and stagnant zones of the internal circulations for a slug with and without a wall film is discussed. The internal circulations could be qualitatively and quantitatively characterized with the help of the PIV measurements and particle tracing algorithm.