X-ray diffraction studies have been made of concentrated KOH and KCl solutions. The radial distribution function for KOH indicates that the hydration number of K+ is 4, in agreement with the theoretically calculated value of Bernal and Fowler. It appears that K+ which is close in size to the H2O molecule enters substitutionally into the pseudostructure of H2O without disturbing the structure to any marked extent. The quantitative evaluation of the data on KCl solutions is more difficult because the first peak in the distribution function is not symmetrical. However, the results indicate that the Cl— ion disrupts the H2O structure and leads to an increase in the number of H2O nearest neighbors.