The role of ions in a tropical river water (Neyyar, Kerala) was assessed. Rock weathering is the dominating mechanism controlling the major ion chemistry of Neyyar river. The dominance of Cl-HCO3-Na during monsoon indicates that geology plays a major role in controlling water chemistry. The presence of mineral varieties of quartz, feldspars, pyroxene, biotite, etc., in the Pre-Cambrian crystalline namely Khondalite and Charnockites, could be the source of major ions. The major ion chemistry of Neyyar river waters show that Na is the dominant cation with lower proportions of Mg and Ca, with HCO3 and Cl as the dominant anions. It belongs to the HCO3 group with significant amounts of Na and Cacations. The dominance of Cl-HCO3-Ca and Ca-HCO3-Cl during post monsoon and pre monsoon respectively indicates that besides geology, the land drainage and anthropogenic activities also control river water. The change in water type observed during monsoon from Na-Cl-HCO3 to Na-Ca-Cl-HCO3 almost from the middle portion may be attributed to anthropogenic pollution. Hence attention is to be paid to take the control measures to prevent the pollution in this stretch of the river.