A preliminary investigation was made in February 1968 of the relationship between the concentration of dissolved silicon and chlorosity in the Vellar Estuary. The Vellar River water and the sea water at the mouth of the estuary differed in concentration of dissolved silicon by about two orders of magnitude and the extent of dilution with fresh water was the dominant influence on the distribution of dissolved silicon in the estuary. Biological effects appear to have been unimportant except towards the mouth of the estuary where the concentration of dissolved silicon was lowest. In the upper estuary, where Vellar River water mixed with water of intermediate chlorosity from the lower estuary, there was some removal (on average not more than 10%) of the dissolved silicon entering in river water. The extent to which further mixing was accompanied by removal is uncertain. The distribution in the lower estuary could be interpreted as indicating either that there was further and more substantial removal, or that there was a considerable contribution of fresh water from irrigation canals which had a lower concentration of dissolved silicon than that in the Vellar River water, or that both these processes occurred. Some implications of these alternative interpretations are discussed.