In this article we investigate the influence of a rectangular microchannel aspect ratio on the liquid-liquid flow. An ionic liquid was chosen as one of the working fluids due to a vast scope of applications. Immiscible ionic liquid-water flow in T-shaped microchannels with 160 mu m hydraulic diameter and aspect ratios equal to 2 and 4 was studied experimentally. Flow pattern maps were drawn and compared for two channels in terms of We*Oh dimensionless number. Parallel flow was shown to prevail for the channel with higher aspect ratio. The influence of channel aspect ratio on plug flow peculiarities was studied in detail. Plug length and velocity were measured at different bulk flow rates. Velocity fields in aqueous plugs were measured using the micro-PTV technique. Velocity circulations were calculated for different flow rates. Total circulation value inside plugs was found to be higher in the channel with the lower aspect ratio. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.