The solubility of C-O-H-Cl-bearing fluids in andesitic melt was investigated experimentally at temperatures of 1050 and 1200 °C and pressure of 200 MPa. The CO2-free andesitic melts produced experimentally contained up to 5.5–6 wt.% H2O and up to 2.5 wt.% Cl at the investigated conditions. The complex non-linear relationship between the concentrations of H2O and Cl in the melt indicates a strong non-ideality of the H2O-salt-bearing fluid phase(s), presumably resulting in the formation of two immiscible fluids. The addition of CO2 to the system (80–800 ppm dissolved CO2 in the melt) has two effects: a simple dilution of the fluid at low bulk Cl concentration and a significant influence on the mixing properties of the H2O- and Cl-bearing fluids related to the enlargement of the immiscibility gap in the fluid at higher bulk Cl content. The first effect results in lower H2O (by ∼ 1 wt.%) and almost constant Cl concentrations in the melt, while the second effect causes an increase in activity coefficients of both H2O and Cl. This effect of CO2 is expected to be most pronounced at low pressures and high CO2 concentrations in Cl-bearing mafic melts due to an enlargement of the immiscibility gap in fluids with decreasing pressure and an increase in proportions of divalent cations.