A multi-syringe flow injection analysis (MSFIA) system coupled with liquid chromatography technique for the simultaneous determination of benzoic and sorbic acids was developed. In order to protect the HPLC column, an on-line sample dialysis method was also implemented. The analyte compounds passed through the dialysis membrane to an acceptor channel (ammonium acetate buffer). Next, the dialysate solution was introduced to the HPLC system. Benzoic and sorbic acids were determined using isocratic separation conditions of reverse phase HPLC with a detection wavelength at 230 nm. The separation was carried out using the mobile phase containing methanol: ammonium acetate buffer (0.05 mol L^-1), pH 4.4 (35:65 % (v/v)), flow rate at 1 mL min^-1 and column temperature at 25 °C. Under optimum condition, the linear range was between 10-100 mg L^-1 for benzoic acid and 10-200 mg L^-1 for sorbic acid. The detection limits for benzoic and sorbic acids were 3.4 mg L^-1 and 10.3 mg L^-1, respectively with good precision (3.9 % RSD of benzoic acid and 2.9 %RSD of sorbic acid). This proposed method was applied to determine both preservative acid compounds in food with acceptable percentage recoveries (81.8%-107.2%). The developed MSFIA-HPLC with on-line dialysis method is simple with low consumption of solutions and short analysis time. The dialysis membrane can protect the performance of the HPLC column by removing the matrix components of the samples.