Volcanic lake systems derive their gases from four distinct sources. Of greatest importance from a hazard perspective, and those which set these limnic systems apart from non-volcanic lakes, are gases derived directly from magmatic sources feeding the volcano, including CO2, SO2, H2S, HCl, HF and a myriad of minor species. The major gases are acidic in nature, and when dissolved into ground water, lead to the development of aggressively acidic solutions. Hydrolysis reactions wiThenclosing rocks, systematically alter the magmatic gas compositions towards more benign hydrothermal signatures, and this process usually leads to precipitation of permeabilityreducing mineral assemblages. Ground and lake waters carry dissolved atmospheric constituents into these environments, whereas lakes are wellknown biotic environments, whose populations may also leave their mark on solute gas compositions through their normal metabolic processes. Apart from magmatic eruption events, at least two specific hazards are attributable to gases in volcanic lake environments, both of which have been responsible for loss of life near volcanic lakes. Physical and chemical processes extant in systems where magmas lie within 100s of metres of the surface have the propensity to form mineralogic seals beneath the lakes. Such sealing may foster over-pressuring and associated gas-driven phreatic eruptions of the type that has occurred recently at Ruapehu and Raoul Island, New Zealand, often with little or no precursory activity. On the other hand, where magmatic gas sources are deeply-seated, to such an extent where heat decouples from the rising gas stream (principally CO2), conditions are perfect for the formation of cold, gas-stratified lakes. Overturn of such lakes typically leads to violent release of the gas, as has occurred in the Cameroonian Lakes Nyos and Monoun, leading to the deaths of near-by inhabitants. Both situations are endmembers of a continuum of processes operating where volcanoes interact wiThearth’s hydrosphere.