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Phosphonic acid (phosphorous acid), H - PO(OH)2, the active breakdown product in plant tissues of the systemic anti-oomycete fungicide fosetyl-aluminium was investigated in order to assay the stability of the labelled compound. There is a strong and fast isotopic exchange in unbuffered aqueous solutions of 2H - PO(OH)2, and therefore 3H - PO(OH)2, so that Fenn and Coffey's experiments must be reinterpreted because these authors used a stock solution of pure labelled phosphonic acid. However, it is possible to use this last compound for biological experiments under the following conditions: (1) Keeping the dry sodium salt 3H - PO(ONa)2. 5H2O in small flasks (100 mg, 37 MBq each) in a vacuum desiccator for individual use without fractionation; (2) using the compound at neutral or slightly acidic pH. [ABSTRACT FROM AUTHOR]