A study has been made of the adsorption of isoquinoline, thiourea and urea at the mercury-aqueous electrolyte interface. Differential capacities of these systems were measured for a range of compositions and, for thiourea, over a range of temperatures. An automatic a.c. bridge technique was developed and a logging system was used to output all information onto tape for off-line computational analysis. The measurements were subjected to the customary thermodynamic analysis in terms of inner layer capacities, surface pressures, surface excesses, and inner layerpotential drop. Particular consideration was given to questions of congruence and to the fitting of the results to a number of isotherms. Only a preliminary study was made of the system isoquinoline-KOH-H20 because during the research the author became aware of the extensive measurements of Gierst on the same system. Nevertheless, some new features are described. An extensive study of urea-NaCl-H20 is only briefly reported, partly out of consideration for the size of the thesis but also because the results are closely similar to those reported by Parsons. The study of the system thiourea-KC1-H20 is reported and analysed in full. The adsorption of the two solutes takes place almost independently of each other. The largest influence, however, was that of thiourea which is strongly adsorbed at a fixed orientation to the surface. Some evidence was detected of co-adsorption thiourea and chloride ions and also of a phase transition at anodic potentials. The influence of temperature and of anion-type on these second order features was marked. A separate, systematic study was made of the entropies of adsorption of thiourea and chloride ions. Consideration was given to the formulation and resolution of the various excess entropies. That of thiourea STS was a linear function of r.r and allowed the evaluation as a function of charge of the residual entropy of formation at constant rT. This residual quantity displays a distinct maximum at q - -3±1 uC/cm2, i.e. at the same charge density as for other aqueous solutions and suggests the adsorption of water takes place largely independently ofother solutes and without cluster formation.