This study was carried out to determine the chemical composition of cave water in Zanzibar Island, part of Tanzania. It was done by studying major cations (Ca2+, Mg2+, Na+, K+), and major anions (HCO3-, SO42-, Cl-), in the water as indicators. Overall results indicated that, the cave water was influenced by marine and weathering contribution. However, weathering was more pronounced at Chomvi ndogo, Chomvi kubwa, and Miza wa Miza caves where it was more than 87% compared to Kilindi, Choweni and Makutani caves where it was less than 77%. Therefore marine contributed more in these last three caves. The trend for concentration contributed from marine water was similar to all sampling sites which was K