The standard pH of 0.05 mol.kg(-1) potassium tetraoxalate was determined using the calculation method recommended by IUPAC (Pure Appl. Chem. 2002, 74, 2169-2200) at various temperatures from new Harned cell data and literature data. New data were obtained for 0.05 mol.kg(-1) potassium tetraoxalate solution containing up to 1 mol.kg(-1) KCl at temperatures of (5, 15, 25, 35, and 45) degrees C. One step in the calculation method of IUPAC is a linear extrapolation of the experimental quantity p[a(H+)gamma(Cl-)], where a refers to the activity and gamma refers to the activity coefficient, to zero molality of KCl. The possibilities of using more concentrated KCl solutions in this determination with nonlinear extrapolations were also investigated.