Abstract To better understand the cause of the Nyos gas disaster of August 21, 1986, we conducted geochemical and limnological surveys in October 1986, of three lakes (Nyos, Monoun and Wum) which are located in the Cameroon volcanic zone that is characterized by a prevalence of young alkaline basalts and basanitoids. Lake Wum was studied as a non-active control: CO 2 is dissolved in significant concentrations (about 1 5 of saturation) in gas-active lakes (Nyos and Monoun), but is virtually absent in Lake Wum. Stable isotopic ratios of total dissolved carbon ( δ 13C= −3% for Nyos and −5.5% for Monoun) and of helium (5.7 R atm for Nyos and 3.6 R atm for Monoun) indicate a mantle origin of these gases. However, SO 4 2− and Cl concentrations are found to be very low. Concentrations of dissolved chemical species like Fe 2+, Mg 2+, Ca 2+, and HCO 3 − are high in the two gas-active lakes, whereas they are very low in the gas-inactive lake. High salinities in the gas-active lakes are probably due to dissolution of indigenous mafic rocks and transported lateritic soil in acidic CO 2-rich, warm water. The gas-active lakes are characterized by increasing temperature and salinity with increasing depth, indicating an active influx of heat and dissolved materials at the bottom. Density estimates show that the lake water is stably stratified in spite of the inverse temperature profile of the lakes, on account of dissolved chemical species. The concentrations of dissolved carbonate species (CO 2(aq) and HCO 3 −) are positively correlated with those of ionic dissolved species, indicating their common occurrence in the bottom water. The August 1986 gas bursts from Lake Nyos were most likely caused by rapid exsolution of dissolved CO 2 within the lake; an explosive process such as a phreatic eruption or a CO 2 gas-jetting from beneath the bottom is unlikely because of low concentrations of Cl − and SO 4 2−, no oxygen isotopic shift, low turbidity, and no reported perturbation of the bottom sediments. Exsolution of CO 2 bubbles could occur if CO 2-saturated bottom water was displaced upwards by an increased influx of high salinity water from the bottom during the rainy season. Exsolution of CO 2 at the upper layers was possibly accelerated by upwelling of a two-phase fluid (CO 2 bubbles and solution), a mechanism known as a pneumatic lift pump, resulting in discharge of a large amount of CO 2 gas. The H 2S concentration in the gas cloud must have been kept far below the lethal level because of a high Fe 2+ concentration of the lake water.