In this work, the high-temperature production of the imidazolium-based ionic liquid 1-butyl-3-methylimidazolium bromide ([BMIM]Br) by means of micro reaction technology was investigated. This reaction is strongly exothermic and a two-phase liquid/liquid system is formed. Heat and mass transfer limitations can, therefore, have a negative effect on product quality and reaction rate. Here we demonstrate that, even at harsh thermal conditions, the ionic liquid, [BMIM]Br, can be produced at high quality. Side product formation, which usually occurs at elevated temperatures, leading to strong discolorization, can be avoided by enhancing mass transfer in a capillary slug flow. This was done by adjusting the flow rate so that the reaction regime shifted from strongly mass transfer limited to kinetically controlled. Here, the Hatta-number had been shown to be a Suitable measure for defining the minimal flow rate. At 145 degrees C, [BMIM]Br could be produced continuously within a residence time of 1.3 min and at a space time yield of 1240 kg L(-1)day(-1). This successful approach for continuous ionic liquid production with up to a thousand-fold space time yield compared to batch processes could be easily transferred to other imidazolium-based ionic liquids.