Previous measurements of the composition of the double layer in mixed solutions of HCl + BaCl2 were incorrectly calculated. Recalculation leads to results in agreement with Gouy-Chapman theory within the large scatter. More precise measurements have been made by the capacity method in dilute aqueous solutions of KCl + MgCl2 at 25°C. Adsorption of K+ and Mg2+ ions is in agreement with that calculated by Gouy-Chapman theory but both are slightly greater than predicted by the simple theory. This is similar to behaviour observed in dilute solutions of single salts and seems to be more dependent on the nature of the anion.