Eight different carboxylate ions were extracted with Crystal Violet into nitrobenzene. The Karl Fischer method was employed to determine the numbers of water molecules coextracted with the carboxylate ions, i.e., their hydration numbers (n(h)) in nitrobenzene. The values of n(h) for the carboxylate ions were little dependent on their chemical structures, being 2.4 +/- 0.2 as the average value.