The study on 10 lakes within the Ethiopian Rift Valley during March-May 1991 covered a range of conductivity (K 25 ) between 286 and 49100 /S cm '. HCO 3 -CO32-and Na + were the dominant ions in all the lakes. Concentrations of K', Cl-and SO4-increased with increasing salinity and alkalinity, whereas Ca 2+ and Mg 2 + decreased. Comparison of these data with previous records showed that a ten-fold dilution of total ionic concentration occurred over 30 years in Lake Metahara and about three-fold increase occurred over 65 years in Lake Abijata. Concentrations of soluble silica were generally high (12-222 mg SiO 21 -1) and increased with increasing salinity, except for Lake Chamo which showed SiO 2 depletion (to < 1 mg SiO21 -1) over the past three decades.The relationship between ionic concentration and phosphorus was irregular although high phosphorus concentrations generally corresponded with increasing salinity. Fitting data to the Dillon & Rigler (1974) chlorophyll a -total phosphorus relationship suggested that lakes Zwai, Awassa and Chamo are phosphorus-limited, whereas others have surplus phosphorus.