To use ionic liquids in pH-dependent reaction and separation processes, pH control is necessary. The use of traditional mineral acids poses a challenge due to the nonvolatile nature of both the ionic liquid and the mineral acid/salts, requiring additional separation steps. The use of a volatile acidification agent, such as CO2, can be beneficial and is the subject of this work. The pH of 1-ethyl-3-methylimidazolium acetate ([emim][Ac]), a basic ionic liquid, and water mixtures is measured under low CO2 pressure. Water content is varied from 0 to 25% (w/w), and CO2 pressure is varied from 0 to 1.5 bar. The effect of temperature is also examined for the 25% water and [emini][Ac] mixture. Results are compared to those obtained using an alternative volatile acid, acetic acid. The pH of [emim][Ac] and water mixtures is found to decrease with increasing water content and increasing CO2 pressure, while temperature has a minimal effect. Two mechanisms are proposed to explain the pH behavior of both pure [emim][Ac] and [emim][Ac]/water mixtures under CO2 pressure. Low-pressure CO2 is capable of providing mild and reversible acidification and offers a valuable tool for pH control of ionic liquids.