Chlorinity in the Cochin Backwater (a tropical estuary) ranges from 0 to 20%, during the year. Determinations of chlorinity by the conventional titration method and those derived from conductivity measurements using the International Oceanographic Tables differed largely in the range 4-9%,. When the available data on chlorinity, salinity and electrical conductivity from the literature were compared, these were found to differ in a similar manner. The main reason for this difference seems to be that the water from the Baltic Sea has been used for determining the conductivity ratios in the lower range. Probably the calcium-rich river water flowing into the Baltic Sea gives a greater conductance than the sea water diluted by distilled water or by the natural rain water.