A new GC/MS method for the determination of diacetyl and 2,3-pentanedione was investigated. Diacetyl and 2,3-pentanedione were derivatized with 1,2-diaminobenzene to form 2,3-dimetylquinoxaline and 2-ethyl-3-methylquinoxaline. respectively. The amounts of formed 2.3-dimetylqu:inoxaline and 2-ethyl-3,.methylquinoxaline were proportional to the concentrations of diacetyl and 2,3-penianedione present in the sample. 2,3-Dimetylquinoxaline and 2-ethyl-3-methylquinoxaline were extracted by solid-phase extraction (SPE) columns and determined by gas chromatography using a mass selective detector. This method was applied for the determination of diacetyl and 2,3-pentanedione concentrations in beer. Extraction by SPE columns proved to be very simple and reliable. The method can be used for simultaneous determination of diacetyl and 2,3-pentanedione concentrations in beer in a great number of beer samples.