A critical assessment has been made of the processes of supply and removal of dissolved silicon in the ocean. The only sources of importance appear to be continental drainage (supplying 4.3 × 10 14 g SiO 2/yr), Antarctic weathering and migration from sediment pore waters. The magnitudes of the last two processes are uncertain but there is evidence that they may add significantly to the river input. The total input appears to lie uncertainly within the range of 5.12 × 10 14 g SiO 2/yr. Estimates of removal in estuarine mixing processes (less than 1 × 10 14 g SiO 2/yr) and in pelagic siliceous oozes (less than 2 × 10 14 g SiO 2/yr) suggest that deposition by these processes may not balance the input. Other removal processes could include biological deposition in coastal waters, but the hypothesis that some removal in the sea occurs by inorganic processes, such as reverse weathering reactions, cannot be discounted.