A semiautomatic measurement system has been developed for investigating the electronic structure of the interface between an insulator and a semiconductor. The associated microcomputer possesses advanced software which leads to simple operation particularly when used in the real time mode. An attractive feature of the technique is that admittance data are evaluated in the voltage domain using a modified version of the Simonne method. The system has been used to investigate the effects of gases on the interface state spectrum of MIS devices; the action of hydrogen upon the palladium-silicon dioxide-silicon system receiving the most attention. An alternative insulator to silicon dioxide has been considered, namely organic Langmuir films. Overall there has been little detectable change in the surface state density, although results with different semiconductors appear to be more encouraging.