Comprehensive studies of phase equilibria of CO2 in various aqueous electrolyte solutions are important to many applications such as CO2 capture, storage and sequestration. Microfluidics (MF) can be a valuable tool for investigation of CO2 mass transfer and solubility in electrolyte solutions, due to its superior mass and heat transfer, well-defined gas–liquid interfacial areas and the ability to vary reagent concentrations in a high-throughput manner. Uniformly sized bubbles of CO2 were isothermally generated in co-flowing liquid streams with different compositions within a MF platform and the evolution of the bubble lengths were followed visually. The solubility of CO2 in the solutions was obtained from a comparison between the terminal and the initial plug sizes. Measurements of CO2 solubility in aqueous single electrolyte solutions (NaCl, Na2SO4, KCl, K2SO4, MgCl2, MgSO4 and CaCl2) and aqueous mixed-electrolyte solutions at 298.15 K were performed in a microfluidic platform. The results of this work provide essential data for the development of comprehensive thermodynamic models for CO2 solubility in geothermal brine solutions.