This study focuses on the determination of factors that control high fluoride groundwater and salinization in basement aquifers of a semi-arid area in central Tanzania. Water samples were collected from productive dugouts, shallow wells and boreholes. The presence of 18O, 2 H, and major cation and anion chemistry, including fluoride, was determined with the use of standard analytical techniques. A sodium-bicarbonate type of water dominates the study area. The 18O - 2 H relationships suggest that the infiltration of water and groundwater has undergone evaporation while in some cases preferential flow mechanisms are suggested. Shallow wells and dugouts were found to have higher concentrations of both fluoride and chloride and this fact is partly attributed to the effect of evapotranspiration. The main mechanism for fluoride input into groundwater as well as salinization is attributed to the leaching of surface and near-surface soil salts, especially trona. No clear relationship was observed between the depth of wells/boreholes and fluoride concentration. Similarly, there is hardly any clear relationship between the fluoride concentration and the geomorphology of the study area. A more detailed investigation is needed to determine the relationship between sampling depth, fluoride concentration and salinization.