Diacetyl can be determined by adsorptive stripping voltammetry after derivatization with o-phenylenediamine. The method may be applicable to the determination of diacetyl in different foods, being a good alternative to other analytical methods. In this work an on-line automated analytical system for diacetyl determination in beer is described. A hanging mercury drop electrode voltammetric flow detector was used, and the analyte was determined without the traditional deoxygenation procedure. The method was successfully applied to the determination of diacetyl during beer fermentation and in the final product. The automation strategy used was based on a flow network similar to those used in multicommutated flow systems, with a pervaporation unit used for diacetyl separation. The developed system was tested in real conditions in the monitoring of brewing processes. The results obtained were similar to those obtained with the usual GC-ECD methodology in the 5-600 ppb range. The analytical rate of the developed method is about 12 determinations/h.