The isodielectric rule is a corollary of the theoretical equation KΛ=(4πNa3/3000) exp (e2/aDkT) derived for primitive model electrolytes. Real systems, especially those involving small ions in structured solvents, do not follow the rule because energies of short range system-specific interactions contribute to Δg, the difference in free energy between paired and unpaired ions. Δg=−kT ln (KΛ/Vm), where VM is the molar volume (in liters) of the solvent. The thermodynamic constant Ka=ap/(a±)2, where ap is the Lewis activity of the pairs and a± that of cation and anion; for ideal solutes, activity=mole fraction. Incidentally, ln KΛ calls for an undefinable operation: only pure numbers have logarithms.