A method has been developed to separate and quantify phenolic acids (gallic-, caffeic-, p-coumaric-, o-coumaric and ferulic acid) and their microbial conversion products by means of high performance liquid chromatography (HPLC). The separation was performed using a Inertsil ODS-3, 3 µm (4.6 * 150 mm) column. Acetonitrile and 50 mM phosphoric acid formed the mobile phase. Quantification was done at three different wavelengths. The method has been validated by measuring the linearity, the limit of detection, the range, the limit of quantification and the reproducibility