The concentration of dissolved silicate in Southampton Water in December 1966 ranged from 320 to 3320 μg Si/l, and showed a highly significant inverse correlation with salinity; the values fitted satisfactorily to a linear relationship. The distribution of dissolved silicate, in the absence of any significant biological activity, was governed principally by the mixing processes in the estuary; there was no evidence of removal of dissolved silicate by inorganic processes. The concentration of particulate silicon was on average more than twice that of dissolved silicon.