Ionic liquids (ILs) are promising new solvents, e.g. for catalysis and separation processes. Currently, ILs are produced in batch reactors at comparatively high prices. So improved and continuous reactors are probably provitable. Reaction engineering aspects of the exothermic IL-synthesis are exemplarily discussed for ethylmethylimidazole ethylsulfate ([EMIM][EtSO4]), formed by liquid phase alkylation of methylimidazole with diethylsulfate. Simulations based on the experimentally determined kinetic data show that an adiabatic loop reactor with an external heat exchange is the best choice, e.g. compared to expensive multi-tubular reactors, which are cooled intensively to avoid a runaway. The unfavourable dilution of the reactants in the loop reactor is outbalanced by the higher inlet temperature, which leads to a higher mean reaction rate (compared to multi-tubular reactors) and a much smaller reactor without exceeding the maximum temperature of 100 degrees C (coloration of IL; long-term thermal stability). In addition, the design and operation of a loop reactor is straightforward and less complex. Compared to batch reactors, the hold-up is by a factor of 1000 lower, which is particularly advantageous for toxic reactants (here diethyl sulfate). The results are beyond synthesis of [EMIM][EtSO4] instructive for other ILs, and probably also for other exothermic reactions with a temperature limit. (c) 2006 Elsevier Ltd. All fights reserved.