The standard partial molar entropy of the aqueous tetrabutylammonium cation, not known previously, has now been obtained, based on the molar entropy of two of its crystalline salts, the iodide and the tetraphenylborate, recently determined experimentally for this purpose. The calculation required also published molar enthalpies of solution and solubilities of these two salts as well as of the perchlorate. The choice of the anions depended mainly on the limited solubilities of the examined salts in water, facilitating the estimation of the relevant activity coefficients. The result is S-infinity(Bu4N+, aq) = (380 +/-20)J . K-1 . mol(-1) at T = 298.15 K, on the mol. dm(-3) scale and based on S-infinity(H+, aq) = (-22.2 +/- 1.2)J . K-1 . mol(-1) (yielding the 'absolute' value). The molar entropy of this cation in the ideal gas standard state, S degrees(Bu4N+, g) = (798 +/- 8)J . K-1 . mol(-1) then yielded the molar entropy of hydration Delta S-hyd(infinity) (Bu4N+) = (-418 +/- 23)J . K-1 . mol(-1). (C) 2008 Elsevier Ltd. All rights reserved.