The objective of this study was to investigate the isotopic composition of oxygen bound to phosphate (δ(18)O-PO(4)) in different phosphorus (P) pools in plant leaves. As a model plant we used soybean (Glycine max cv Toliman) grown in the presence of ample P in hydroponic cultures. The leaf blades were extracted with 0.3 M trichloroacetic acid (TCA) and with 10 M nitric acid. These extractions allowed measurement of the TCA-soluble reactive P (TCA P) that is rapidly cycled within the cell and the total leaf P. The difference between total leaf P and TCA P yielded the structural P which includes organic P compounds not extractable by TCA. P uptake and its translocation and transformation within the soybean plants lead to an (18)O enrichment of TCA P (δ(18)O-PO(4) between 16.9 and 27.5‰) and structural P (δ(18)O-PO(4) between 42.6 and 68.0 ‰) compared with 12.4‰ in the phosphate in the nutrient solution. δ(18)O values of phosphate extracted from soybean leaves grown under optimal conditions are greater than the δ(18)O-PO(4) values of the provided P source. Furthermore, the δ(18)O-PO(4) of TCA P seems to be controlled by the δ(18)O of leaf water and the activity of inorganic pyrophosphatase or other pyrophosphatases.