We show a technique to measure alkylcarbonic acid (ACA) formation and dissociation constants in CO2-expanded methanol, ethanol, and benzyl alcohol at 25 degrees C. In these systems, CO, reacts with alcohols to form an in situ, self-neutralizing acid that exists under CO, pressure. UV/vis spectroscopy is a noninvasive method to assay proton concentration, and the resulting values of this proton concentration yield effective equilibrium constants, the product of the formation, and dissociation constants. To separate the acid dissociation constant from the formation equilibrium constant, we employ a method using mass balances. From this, we find that methylcarbonic acid has a pK(a) of 5.7 and the relative ACA strengths are methyl > ethyl > benzyl. This work provides key parameters to design processes and expanded solvents that fully exploit the environmental and processing benefits of ACAs.