Determination of data adequacy for detection of long-term salinity changes was an important task in the revision of the South African National Chemical Monitoring Programme (NCMP). The NCMP has been running for more than 30 years with several hundred active monitoring sites. Twenty-five sites on major rivers had sufficient continuous data for the estimation of salinity changes over a 25-year period and statistically significant upward or downward trends occurred at 17 of the 25 sites. Most sites were too far apart for detailed analysis of whole river systems, though an upward trend is apparent in the Lower Orange River and a downward trend in the Great Fish River. Salinity in the Tugela River remained stable, well below the 70 mS m( - 1) guideline for drinking water. The results underline the importance of long-term data sets for assessing and managing aquatic systems and provide the impetus to continue building and maintaining long-term sampling programmes.