An extensive field investigation to study mixing and dispersion in Southampton Water entailed the installation and maintenance, over a period of one year, of three recording salinometers in the lower reaches. In order to relate salinity recordings to cross-sectional behaviour, regular fortnightly boat traverses were also undertaken during the same period. Salinity monitoring was carried out using commercially-available salinometers providing outputs of conductivity and temperature on strip-chart paper. In order to improve the accuracy of the recordings, extensive electronic modification of the instruments was necessary. The chart records were digitised using a modified sonic device. The raw data were processed by computer to produce quarter-hourly values of salinity and temperature on magnetic cartridge. Tidal heights were similarly obtained from autographic recordings at Calshot Spit. Multiple linear regression analysis has shown that tidally-averaged salinity is highly dependent upon freshwater discharge and virtually insensitive to tidal range. Vertical stratification was found to be slight except at maximum discharge. Contrary to earlier findings, salinity is normally lower on the westside of the estuary at the Test and Itchen confluence. The transverse gradient was found to be highly correlated with the lateral component of wind stress. Steady-state salinity profiles were used to compute values of effective longitudinal dispersion coefficient. These were found to increase with increasing discharge and with distance towards the estuary mouth. A one-dimensional tidally-averaged water quality model of Southampton Water and its major tributaries was developed using an implicit finite difference technique. Important features of the model include a predictor-corrector method of solution, variable upstream boundaries, flow accumulation along the reach, and self-adjustment of dispersion coefficients to varying discharge. The model was shown to faithfully reproduce salinity intrusion and gave reasonable concentration predictions for an effluent undergoing first-order decay. In addition to the main research programme, intensive observations of salinity and current velocity were made throughout a tidal period across the mouth of Southampton Water to determine the neap tidal exchange ratio. By applying a salinity tracer method, the neap exchange ratio was computed to be 32 ± 4 per cent.