Reactive phosphate (RP) fluxes over sediment in an eutrophic bay were obtained under different conditions. The results were assessed together with water column, sediment solid, and liquid-phase parameters. The RP fluxes ranged from 0 to 1,300 mu gatP x m(-2)d(-1) day depending on the sediment site and other variables. At stations with relatively high net RP fluxes, the clay fraction was > 3% and disproportionate with salinity, which explained why the RP effluxes decreased by inhibition of the organic coating on particulate matter with increased SO4 (-2) ions and good competition of RP anions with SO4 (-2). The RP flux in Fe/RP ratios > 2 in surface water decreased to approach a minimum asymptotically, which was thus defined with a hyperbolic curve to determine the maximum rate of the RP flux (J(RP)). The [RPporewater](initial)/[RPambient] (initial) ratios were suggested to govern the net RP flux regionally, so the critical value where the RP flux turns positive could indicate eutrophication with Chl-a values < 500 mu g/g. So, this study investigated the phosphate fluxes between sediment and water column and determined the factors influencing the RP fluxes in the inner bay of Izmir.