EVo calculates the speciation and volume of a volcanic gas phase erupting in equilibrium with its parent magma. Models can be run to calculate the gas phase in equilibrium with a melt at a single pressure, or the melt can be decompressed from depth rising to the surface as a closed-system case. Single pressure and decompression can be run for OH, COH, SOH, COHS and COHSN systems. EVo can calculate gas phase weight and volume fraction within the system, gas phase speciation as mole fraction or weight fraction across numerous compounds, and the volatile content of the melt at each pressure. It also calculates melt density, f02 of the system, and more. EVo can be set up using either melt volatile contents, or for a set amount of atomic volatile which is preferable for conducting experiments over a wide range of fO2 values.