Several mineral constituents can be detected by UV–visible spectrophotometry either directly (nitrate, nitrite, sulfide, hypochlorite, and chromium) or indirectly after a phtooxidation or a complexation step (ammonia, total nitrogen, iron, copper, mercury, etc.). Among the different mineral constituents, nitrate and phosphate ions are of primary importance regarding their impact on freshwater and coastal water quality (risk of eutrophication). The direct detection of nitrate ion in freshwater is one of the main applications of UV spectrophotometry and high-frequency measurement is even possible with available commercial devices for temporally and spatially nitrate monitoring. For phosphate, the measurement is carried out with a colorimetric procedure, but simple analyzers are also commercially available for high-frequency measurement. Other interesting applications for the detection of mineral constituents in water are also presented.