Effects of iron compounds on methanogenic fermentation the water polluted with fatty acids were studied. A natural readily available source of iron applicable to biological treatment of liquid wastes was searched for. A positive effect of iron on the methanogenic fermentation of fats and their degradation products--long-chain fatty acids--in aqueous media was demonstrated. It is recommended to add iron-containing clay, as an inexpensive and easily available iron source, in amounts providing the binding of the long-chain fatty acids present in wastewaters.