Microchannel two-phase heat exchanger geometries have found widespread adoption in energy systems due to their potential for high wall-to-fluid heat transfer coefficients. However, flow maldistribution and poor bulk mixing may limit performance in applications where inter-phase transport is critical, such as in absorption refrigeration and water distillation. Mini-gaps with pin-fin arrays have been highlighted for their potential to enhance transport in these applications by breaking up large liquid and gas structures, but two-phase flow mechanics are still poorly understood in such geometries. To address this need, an experimental study of adiabatic (air-water) two-phase flow is performed in horizontal mini-gaps with and without pin-fin arrays (2.5 × 95.3 mm cross section). High speed shadowgraphy and pressure drop measurements are collected for gas flow rates of 0.1 − 10.0 l min^-1 and water flow rates of 0.05 to 2.82 l min^-1. A parallel array of pressure transducers and selectable gas flow meters with varying ranges are employed to ensure high accuracy over the full range of experimental conditions. Visualization data is applied to form flow regime maps. Results from this project can help inform the design of next generation microscale heat exchangers.