This paper describes the use of Laser-induced fluorescence to investigate liquid-liquid vertical pipe flow in a closed experimental circuit at very low mixture velocities. The dynamic evolution of annular, mixed and dispersed flows were observed, as well as the associated physical processes such as wave coalescence and the formation of secondary dispersions. The experimental results presented in this paper suggest that as the flow velocity was increased, the flow undergoes a transition from annular to wavy-annular to mixed (or transition) flow, and finally dispersed flow. The results also indicated that secondary dispersions, such as w/o/w and o/w/o, occur in the dispersed core of a mixed flow. In transient flow (in which the flow velocity is decreased), the structure of the flow was very complex and the formation of 'slugs' could occur, which resemble the situation in gas-liquid flows. (c) 2005 Elsevier Ltd. All rights reserved.