The Kattegat region forms a transition zone between the Baltic Sea and the North Sea. Baltic Sea water being brackish and high in yellow substance mixes with saline and dense North Sea water which is either low or high in yellow substance. Yellow substance combined with salinity is most suitable for water mass classification in the region since the Baltic water is characterized by a low salinity in combination with a high yellow substance content, whereas the North Sea water originates either from the Atlantic having extremely high salinities and low yellow substance content or from the German Bight which has an intermediate to high salinity and a very high yellow substance content exceeding the one in the Baltic proper. Measurements of salinity and yellow substance in the Kattegat allow the construction of a yellow substance/salinity diagram which serves the same purposes as the classical temperature/salinity diagram where the latter is not applicable in coastal areas. The yellow substance/salinity diagrams also permit discoloration studies resulting from mixing of Case 1 waters and Case 2 waters rich in yellow substance.