Fluctuation solution theory via direct correlation integrals for corre
lating an predicting the densities and activities of aqueous strong el
ectrolytes up to saturation is described theoretically and from experi
ment. The subtlety, but importance, of the McMillan-Mayer (MM) treatme
nt is contrasted with other thermodynamic developments. Also, formal r
esults are shown which distinguish the effects of solvent compressibil
ity (fluctuations) and electrostatics on property divergences in syste
ms near the solvent's critical point.