A diagnostic technique capable of characterizing interfaces between transparent, immiscible fluids is developed and demonstrated by investigating the morphology of liquid-gas interfaces in an adiabatic two-phase flow through a microchannel of 500 mu m x 500 mu m square cross section. Water seeded with 0.5 mu m-diameter fluorescent polystyrene particles is pumped through the channel, and the desired adiabatic two-phase flow regime is achieved through controlled air injection. The diagnostic technique relies on obtaining particle position data through epifluorescent imaging of the flow at excitation and emission wavelengths of 532 nm and 620 nm, respectively. The particle position data are then used to resolve interface locations to within +/- 1 mu m in the focal plane. By mapping the interface within individual focal planes at various depths within the channel, it is possible to determine the complete liquid-gas interface geometry across the channel cross section in a dynamic flow environment. Utilizing this approach, the liquid-gas phase boundaries of annular flows within a microchannel have been successfully characterized. (C) 2014 Elsevier Ltd. All rights reserved.