The apparent molal heat capacities (phi C-p) of 1 - 1 electrolytes at 25 degrees C have been fitted to the Fitter equation phi C-p + (C) over bar(p,2)degrees + (A(J)/1.21) ln(1 + 1.2I(1/2)) - 2RT(2)[mB(MX)(J) + m(2)C(MX)(J)], where (C) over bar(p,2)degrees is the partial molal heat capacity of at infinite dilution and B-MX(J) and C-MX(J) are empirical constants related to ion-ion interactions. The values of (C) over bar(p,2)degrees of the electrolytes have been used to determine partial molal heat capacities of ions. The coefficients B-MX(J) and C-MX(J) have been combined with Fitter coefficients for enthalpies and osmotic coefficients to develop equations that can be used to determine activity coefficients of these electrolytes from 10 to 70 degrees C and from 0.1 to 2 m to within 1%.