Water-soluble organic acids such as dicarboxylic acids are known to form a significant fraction of organic aerosol mass, yet the chemical composition and interactions between components in an organic acid-inorganic salt mixed particle remain unclear. In this study, phase behavior of different mixing ratios of the salt and organic acids, here 3-methyl glutaric acid and 3-methyl adipic acid, are investigated with respect to their water activity. A microfluidic pervaporation approach is used to study different phase transitions of internally mixed aqueous droplets. Single droplets of varied compositions are trapped and stored in microfluidic wells until dehydration, where both the water content and the solution volume of the droplet decrease slowly with time. The volume is calculated by imaging techniques and correlated with the initial known concentration of the solution to determine concentrations at each time interval. The phase transitions of the droplets with changing concentrations are also observed under an inverted microscope. This study will help determine the concentration at which a mixture droplet, mimicking organic and inorganic atmospheric aerosols, changes phase.