The name bubble-train flow is used to identify the cocurrent flow of gas bubbles and liquid slugs through a capillary. A model was developed to analyze mass transfer between bubbles and liquid slugs during bubble train flow. This mass transfer model, derived from the equations of transport, is based on the fluid flow profiles and has no adjustable parameters. A comprehensive account of the experiments designed to test the model is presented. The single capillary model was used to analyze mass transfer of a passive saline solution introduced as a pulse signal. Theoretical results were used to determine the concentration vs time distribution of the saline solution inside the capillary channels. These results were compared with experimental residence times determined using a conductimetric technique. Very good agreement between theory and experiments was found for single capillaries. The mass transfer model, however, must be modified to take into account macroscopic flow distribution effects before it can be used for the analysis of mass transfer and chemical reaction inside the monolith froth reactor.