The work described in this Thesis consists of investigations of the variation of the electrokinetic potential and charge for the system fused silica/aqueous potassium chloride solution by the streaming potential method. An improved apparatus for the determination of the streaming potentials, used in this work, is described. Attempts have been made to devise a general method of interpreting the results obtained by the streaming potential method using porous diaphragms, even when surface conductance is not negligible. Measurements were made over the temperature range 20 - 50 C and the corresponding electrokinetic potentials and charges were calculated. These data have been used to test theoretical equations which express the variation of the above properties with temperature. The dependence of the electrokinetic potentials and charges on the adsorption energies is considered. Theoretical calculations are given which lead to the elucidation of the processes of adsorption, together with a method of calculating the number of adsorption sites in the fixed layer. Applications of the new experimental techniques and theoretical equations to other systems are also considered.