Abstract: A novel flow injection analysis (FIA) system has been developed for the rapid determination of the volatile acidity of some fermentation products like vinegars and juices. The proposed method is based on the diffusion of volatile acids, mainly acetic acid, across a PTFE gas-permeable membrane from an acid stream into an alkaline stream, and the acids trapped in the acceptor solution are determined online by a bulk acoustic wave impedance sensor based on changes in the conductivity of the solution It exhibited a linear frequency response up to 10 nmol L-1 acetic acid with a detection limit of 50 µmol L-1, and the precision was better than 1% (RSD) at a through-put of 72 hr-1. The effects of operating voltage for the detector, cell constant of the electrode, composition of acceptor stream, flow rates, and temperature on the FIA performance were also investigated.