Sediment oxygen demand (SOD) is controlled by oxygen transfer in a diffusive boundary layer and by oxygen consumption due to microbial and chemical processes inside the sediment. SOD is a function of flow velocity above a sediment/water interface, i.e. shear velocity, and biochemical activity rates. A model was presented for the unsteady response of SOD and the dissolved oxygen (DO) profile near the sediment/water interface. The time required to reach a steady state DO profile and SOD for zero DO inside the sediment initially gets faster as biochemical activity becomes strong. The values of SOD estimated by the model were compared with experimental data. The model can reproduce observed diffusive boundary layer thicknesses. It is also found that values of SOD estimated by the model were of the same magnitude as the observed data.