Knowing the product of fermentation by an anaerobe or facultative anaerobe is an extremely important part of the phynotypic characterization of that organism. The methods of Holdeman et al. (1997), which employ gas-liquid chromatography (GLC) for detection of volatile fatty acids (VFA) and the methyl esters of non-volatile acids (NVA) have been widely used; for details, see tindall et al. (2007). Although NVA and VFA have frequrntly been analysed separately on a single gas chromatography, hight-performance liquid chromatography (HPLC) methods offer the advantages of detaction of NVA without the need for derivatization, detaction of both VA and NVA in a single chromatography run and increased sensitivity for detaction of NVA. Capillary GLC method do involve simultaneous derivatization of VA and NVA, but both can be separated in a single chromatography run. Summaries of these methods are given below.