As group of compounds, fatty acids and fatty acid salts are naturally occurring substances in the environment and are a significant part of the normal daily diet of mammals, birds and invertebrates. Soil has a natural background concentration of fatty acids as a result of plant metabolism and microbial action. As a result of their simple structure, the environmental fate characterisation of fatty acids is well understood; with microbial degradation as the major route of transformation of fatty acids in the environment (through the cleavage of the carbon chain of fatty acids by oxidative chemistry). Fatty acids and their salts provide an excellent substrate for microbial growth, serving both as carbon sources and energy sources. Microbial metabolism of fatty acids has either the effect of either converting degradates to CO2 and ester (if used as an energy source) or converting carbon content of the fatty acid to any of the naturally occurring organic substances produced by soil micro-organisms (if used as a carbon source). In soil these compounds are rapidly degraded in soil by microbial organisms, with the DT50 value for the potassium fatty acid salt estimated to be less than one day, although studies have also indicated fatty acid DT50 values at ~3 days. Fatty acid salt chain lengths up to and including C18 are metabolised under aerobic conditions and, overall, are considered to be readily biodegradable. As a result of rapid degradation these compounds are expected not to persist in the environment. However, salt salts cannot totally dissipate in soil because of the natural content of fatty acids in the soil compartment (EPA738-R-92-0151 and EPA738-F-92-0132). Furthermore, the metabolism of fatty acids is strongly influenced by the compounds physical and chemical properties, in particular their water solubility and adsorption characteristics. For example, pKa values for the free acids heptanoic acid and dodecanoic acid range from 4.4 to 5.3, respectively, and suggest that the ionisation status and, hence, water solubility could vary depending on pH conditions in soil. Unsaturated fatty acids tend to have increased rates of metabolism, although the degree of unsaturation and position of double bonds is not highly significant. Volatilisation from soils is not considered to be a transport process or dissipation route for fatty acids in the environment.