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Abstract: Horizontal flows of two immiscible liquids with the same refractive index, a kerosene-like hydrocarbon and a glycerol–water solution, have been instigated with planar laser-induced fluorescence in a square duct. Four flow regime categories were observed, these being: (1) stratified flow; (2) mixed flow (i.e., two distinct continuous phase regions with droplets in each); (3) two-layer flow, comprised of a dispersed region and a continuous, unmixed region (i.e., oil-dispersed flow over glycerol solution flow and, oil flow over a glycerol solution dispersion); and (4) dispersed flows (i.e., continuous oil phase dispersion and continuous glycerol solution dispersion. The flow can be described as occupying three zones; an oil phase at the top, a glycerol–water phase at the bottom, with a mixed zone between them. The vertical height covered by the mixed zone increased for increasing superficial mixture velocity, and the vertical height of the glycerol–water phase decreased for increasing input oil fraction. At low oil fractions the interface level separating the two phases was not affected by changes to the superficial mixture velocity. However, at higher oil fractions the interface height from the bottom of the channel decreased progressively as the superficial velocity was increased. Higher velocities also gave rise to increasingly fluctuating interface level heights. The mean droplet size increased initially, reached a maximum and then decreased as the oil fraction was increased, and was largest at intermediate and smallest at high superficial velocities. [Copyright &y& Elsevier]