The pK a values for 30 C-acids of the type CH·R 1 ·R 2 ·R 3 -where the R 1 , R 2 , and R 3 substituents are H, alkyl, and aryl, and the most common electron withdrawing groups used in studies of proton-transfer reaction -are given herein. The pK a values have been measured potentiometrically in acetonitrile, using a glass electrode. These values were determined in a buffer solution consisting of a C-acid and its tetrabutylammonium salt, at constant ionic strength (0.001 M), at 25°C. The measurements were done under nitrogen, to prevent oxidation of the anion. The influence of water within the sample of the tetrabutylammonium hydroxide base was negligible. The lowest pK a value of 19.1 was found for bis(4-nitrophenyl)cyanomethane, while the largest pK a s that could be measured were limited by the basicity of the hydroxide ion, and did not exceed 28. A satisfactory agreement between spectrophotometric and potentiometric acidities was achieved.