A headspace gas chromatography-mass spectrometry (HS-GC-MS) method is presented for the simultaneous determination of benzoic acid (BA) and sorbic acid (SoA) in different types of non-alcoholic beverages. Sensitive and reliable results were achieved together with minimising consumption of reagents and samples. Salicylic acid (SalA) was used as internal standard (IS). It was necessary to derivatise BA, SoA and SalA to their methyl esters for HS-GC-MS measurement and extensive optimisation studies for in-vial derivatisation were carried out on the temperature, incubation time, injection time of the loopless HS, as well as on the concentration of sulphuric acid used as a catalyst. Validation studies carried out under optimum conditions after mixing 50 mu L of sample and IS solutions with 200 mu L of 4.5 M sulphuric acid in 22 mL HS vials revealed that the developed method was both very precise (relative standard deviation < 5%) and accurate (average recovery%: 101.0% for BA and 100.4% for SoA). The validated method was applied to a wide range of beverage types and the results compared with the relevant regulation and product label declarations.