High-pressure phase behavior has been measured for CO(2)/methanol/tetramethylammonium bicarbonate (TMAB) and CO(2)/methanol/TMAB/water systems at (25 and 70) degreesC and pressures up to 30 MPa. An increase in methanol concentration results in a decrease in the phase-transition pressure due to the higher miscibility of methanol and salt in CO(2). At higher methanol concentrations, single phase mixtures could be achieved at moderate pressures with up to 70 mol % CO(2). As expected, the addition of water sharply increases the phase transition pressure. The experimental results for the above systems were modeled using the Peng-Robinson equation of state; model predictions were found to be sensitive to adjustable parameters.