This Test Guideline describes a laboratory test method to assess aerobic and anaerobic transformation of organic chemicals in aquatic sediment systems. The method permits the measurement of (i) the transformation rate of the test substance in a water-sediment system and in the sediment (ii) the mineralisation rate of the test substance and/or its transformation products, (iii) the distribution of the test substance and its transformation products between the two phases during a period of incubation in the dark, at constant temperature, and (iv) the identification and quantification of transformation products in water and sediment phases including mass balance.At least two sediments different with respect to organic carbon content and texture are used. Ideally the test substance (one concentration) should be applied as an aqueous solution into the water phase. The duration of the experiment should normally not exceed 100 days, and should continue until the degradation pathway and water/sediment distribution pattern are established or when 90 % of the test substance has been removed by transformation and/or volatilisation. The number of sampling times should be at least six. The study includes: concentration in the water and sediment of the test substance and the transformation products at every sampling time; results from gases/volatiles trapping systems at each sampling time; mineralisation rates; and non-extractable residues in sediment at each sampling point. Half-lives, DT50, DT75 and DT90 values are determined where the data warrant.