Nine sediment-water columns were taken from three stations of East Taihu Lake (about lm in depth) in June, 1997. The overlying water of the cores was filtered (0.45μm), and then the cores were placed in a dark room for 50 days. Air-bubbling was used to keep oxidized conditions in the overlying water and upper sediment, without resuspending the sediment. The room temperature was between 23.5 and 31.2℃ , and the pH value of the overlying water and upper sediment was 7.2±0.2. Concentration of PO₄^3--P in the overlying water was measured every 2 or 3 days to follow the P-release process. In the first 15 days,PO₄^3--P level in the overlying water increased 2 orders of magnitude. But in the following 33 days, the PO₄^3--P level decreased 1 order of magnitude and then stabilized. Such a process was also found in the West Lake, Hangzhou and Gehu Lake, Changzhou. We think it not a normal sediment-P-release process, but more complex as follows. In natural conditions of East Taihu Lake, the sediment is covered by a layer of detritus. Biological degradation is so intensive in this layer that PO₄^3- accumulates to a high level and diffuses to both the overlying water and the sediment beneath it. In the sediment a steep PO₄^3--P concentration gradient forms because of the high diffusive resistance. But in the overlying water, PO₄^3--P is used by aquatic plants and is kept at a low level. We are not able to measure PO₄^3--P concentration in the detritus layer, but an indirect evidence was found in the same lake: a PO₄^3--P concentration gradient from 200mg m^-3 in the upper sediment layer to 80mg m^-3 in the sediment of 18cm depth existed when PO₄^3--P concentration in the overlying water was only 22 mg m^-3. In the core experiment, phosphorous biological cycling was broken when phytoplankton was removed from the overlying water by filtration, and thus, PO₄^3- accumulated in the water quickly. After 15 days, the P-release reaction in the detritus layer was about to end, and P-diffusion from overlying water to the sediment triggered. In East Taihu Lake and the other similar lakes with stable bottom conditions, detritus layer may exist and affect P-release core experiment seriously. In such a situation, it is better to measure internal-P-loading by other means, such as whole lake P-budget and in-situ enclosure experiment.