Fosetyl-Al is rapidly degraded in soil by biological process. The ethyl moiety is highly mineralized (> 70 % in 4 - 16 d) and the corresponding bound residue peaks at 33.5 - 41.3 % after 2 - 4 d (20.4 - 23.9 % after 32 d). Ethanol (max. about 78 % after 1.5 hours) and two unknown degradation products (max. 12.8 and 17.2 % after < 1 h) are derived from the ethyl moiety. Because these metabolites are transient and are only detected for the first day, they are deemed to be not relevant and no further information is required. Phosphorous acid has been identified as a significant degradation product of fosetyl but it has not been precisely quantified (100 % assumed). Data from litterature demonstrate the biological transformation (oxidation) of phosphite into phosphate in soil. Various microorganisms are able to utilize phosphite as phosphorus source and an induction period could be required for the synthesis of an enzyme responsible for the oxidation of phosphite. The provided study on degradation of phosphorous acid in 2 soils shows a rapid initial decrease in the ammonium buffer extractable phosphite fraction followed by a slow decline. Physico-chemical interactions with soil constituents could be involved first (phosphorous acid could be immobilised in soil by formation of less soluble salts and/or complexes) followed by slow oxidation to phosphate. « Bound residue » is the main transformation product (49.4 - 74.2 % after 87 - 117 d). This residue can be fully recovered by fractionation of soil phosphorus as phosphates but it has not been characterised due to analytical difficulties. However, based on the available data, it is reasonable to assume that it could consist of insoluble forms of phosphite and of phosphate derived from oxidation of phosphite. Assuming equilibrium between soluble and insoluble forms of phosphite, no continous accumulation is expected due to transformation of the soluble form into phosphate. Measurement of residues in plants suggests that phosphorous acid which could be present in bound residue has low availability to plants and thus would give rise to little concern. Aluminium is thought to be of minor importance with regard to naturally occuring amounts in soils.