The enthalpy of precipitation of six common univalent cations with the tetraphenylborate anion has been measured at 25.0°C. Reaction entropies range from -8.24kcal/mole for ammonium to -19.7 kcal/mole for silver(I). The entropy of reaction was computed from the above measurements and the corresponding Gibbs free energy changes. A linear relationship exists between ΔF°′ and ΔH°′. The entropy of precipitation of ammonium ions was +5.0 e.u., an unusually large positive value.