Measurements of salinity and concentration of dissolved silicon were made on samples collected in the Test arm of Southampton Water in a number of surveys during 1968. The values show highly significant inverse correlations and fit closely to linear relationships. The distribution of dissolved silicon was controlled essentially by mixing processes. Biological activity in the summer had a negligible effect. For each survey, the concentration of dissolved silicon in the diluting water, as calculated from regression analysis of values over the whole range of salinity, agreed closely with that measured in river water entering the estuary. These findings indicate that there was no appreciable removal of dissolved silicon by inorganic processes. The Hamble Estuary was investigated on one occasion. Mixing processes were again found to be the primary factor affecting the distribution of dissolved silicon but the results suggested that a small fraction (not more than 10%) of the dissolved silicon entering in river water was removed inorganically.