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.