Seasonal variations of sodium, potassium and magnesium in the backwater of Cochin were studied during the four seasons of 1973-'74. Na/Cl ratio of the estuarine water closely follows that of sea water through the various seasons. Monsoon rains were found to influence the Mg/Cl ratio of the estuarine water, the general trend being toward an increase in the ratio with lowering chlorinity. The value of K/Cl ratio was always higher for the estuarine water compared to sea water and the ratio increases with decrease in chlorinity. In all seasons the observed amounts of potassium for various samples were found to have a linear relation to chlorinity.