ON 21 August 1986, a massive release of carbon dioxide from Lake Nyos in Cameroon killed about 1,700 people. A similar event occurred on 15 August 1984 at Lake Monoun, also in Cameroon. It was suggested(1-5) that the CO2 released was initially dissolved in the hypolimnion (dense lower layer) of the lake, and was released by eruptive outgassing. Because of its violence, the Nyos outburst was at first thought(6) to have been volcanic, but undisturbed sediments and other evidence indicate that no large volcanic eruption occurred(7-9). Recent experiments(10,11) have shown that decompression of CO2-saturated water is able to power explosive eruptions. Here I analyse the dynamics of CO2-driven lake-water eruptions by deriving an equation of state for gas-liquid mixtures and using it to integrate the Bernoulli equation, which describes the dynamics of the bubbly flow. I find that under certain conditions these eruptions can be violent : the lake-surface exit velocity of an initially gas-saturated water parcel may reach 89 m s(-1) for Lake Nyos and 51 m s(-1) for Lake Monoun. The dynamics are similar to those of water-driven volcanic eruptions, which are also powered by gas exsolution from a liquid.