​The contribution of volcanic lakes hydrogeochemistry in volcanic surveillance is a very recent area of study when compared with other techniques applied in voleanic monitoring. The study of waters in contact with volcanic fluids allows a better view of these systems. Studies on volcanic lakes, where the condensation of volcanic gases or thermal seepage occur, have called the attention after the sudden CO₂ release in Monoun lake (1984) and, most importantly, in Lake Nyos (1986) in mainland Cameroon.After that, scientific contributions on this subject have steadily increased. In the Azores, namely in San Miguel island, some volcanic eruptions located in volcanic lakes induced transformations in their systems. Nevertheless, hydrogeochemical studies in these places in the Azores Archipelago are very rare, because studies made in lakes were mainly phocused on eutrophication problems. A representative set of 30 lakes from the Azores Archipelago (Portugal) has been studied to overcome the lack of data on their volcanic setting and on water geochemistry of these lakes. The physical and chemical elements analyzed were: pH, temperature, electrical conductivity, total dissolved solids (TDS), total and dissolved CO₂, Na⁺, K⁺, Mg²⁺, Ca²⁺, HCO^3-,CI^-, SO₄^2-, and SiO₂. With the analytical results it was possible to identify significant increases in concentration of different elements, especially total CO₂ in the hypolimnion. Besides the contribution of organic matter degradation to the CO₂ increase, there is t​he possibility that part of this increase is related with thermal water seepage and volatile discharges in these lakes (Sete Cidades, Fogo and Furnas complex).