The electrical conductivity of single crystals of potassium chloride was measured as a function of temperature, and as a function of the amount of strontium chloride or potassium carbonate impurity. The results were analyzed by using a least-squares curve-fitting computer program in which the basic assumption was that conductivity was due entirely to the motion of separated anion and cation vacancies (Schottky defects). Coulomb interactions between charged defects were allowed for by using Debye–Hückel theory. With the exception of the conductivity of pure crystals very near the melting point, experimental conductivity measurements agreed with values calculated from the following parameters: Schottky-defect formation enthalpy hs = 2.59 eV, entropy ss = 9.61k; positive ion vacancy migration enthalpy Δh1 = 0.73 eV, entropy Δs1 = 2.70k; negative ion vacancy migration enthalpy Δh2 = 0.99 eV, entropy Δs2 = 4.14k; association of divalent cations and positive ion vacancies enthalpy χ = 0.58 eV, entropy η = 1.30k.