This report deals with the results of a study on the degradation of halogenated aromatic hydrocarbons in anoxic sediment-water systems. First of all several transformation processes are shown to take place, leading to formation of a variety of products. In this a clear distinction could be made between abiotic and strictly biotic transformation processes, taking place after acclimation periods of varying length. Quantitative Structure-Activity Relationships (QSARs) are developed relating the disappearance rate constants of halogenated aromatic hydrocarbons in anoxic sediments to readily available molecular descriptors. The correlations are based on disappearance rate constants for the reductive transformation of approximately 45 halogenated hydrocarbons. A batch method was used in which the compounds were incubated in a standard water-sediment system and periodically analysed. Subsequently the pseudo-first order reaction rates were correlated to several molecular descriptors. The best correlations were obtained using the following descriptors: 1. The carbon-halogen bond strength. 2. The summation of the Hammett sigma and the inductive constants of the additional substituents. 3. The steric factors of the additional substituents.