Magnesium ions decrease the activity of divalent organic anions much more than the normal decrease produced by sodium ions. The effect is very large with short chain acids, particularly oxalic acid. The addition of sodium or potassium ions produces a marked decrease in the effect of magnesium diions on the activity of oxalate diions. Quantitative data on 0.005 molar solutions of oxalic diion over a wide range of concentrations of MgCl2 and of NaCl (or KCl) show that the following equation is obeyed: See PDF for Equation where A is an empirical value dependent on the concentration of oxalate diion (0x=). This equation has been shown to hold down to zero ionic strength of Na+ and K+, and hence to be valid in the physiological range. These observations are of biological interest since the activity of proteins should (like oxalic acid) show a similar antagonism.