Conventional ion selective electrodes are briefly reviewed, with particular reference to the pH sensitive glass electrode, and the benefits which might result from the application of microelectronic techniques to electrode manufacture are noted. It is shown that microelectronic 'transducers' may conveniently be classified into two general categories, described as the 'potentiometric' type and the 'field effect' type, and certain constructional and operational advantages of the former type are suggested. The theory of membrane potentials is critically reviewed and the relationship between such well known phenomena as the 'Donnan potential', the 'liquid junction potential' and the 'glass membrane potential' is discussed. A model is proposed for the operation of the 'Ion Sensitive Field Effect Transistor', (a transducer of the 'field effect' type) by drawing upon the theories of the glass electrode and the conventional 'Insulated Gate Field Effect Transistor'. The-fabrication of 'field effect' type devices is described and the results of measurements on them are reported. It is noted that a clear understanding of the mechanism and stability of solid state contacts to ion selective materials is necessary for the development of sensors of the 'potentiometric' type. To this end, an experimental structure has been devised to allow measurement of the pH sensitivity and stability of metal connected glass electrode cells. Severe problems are caused by electrical leakage effects but a satisfactory structure has been achieved and might have application in the manufacture of conventional glass membrane electrodes. The results of measurements on metal connected devices show that Nernstian pH responses are obtainable and that cell potentials are fairly stable over periods of several weeks. Improvements to the measurement techniques are required in order to investigate further the long term stability and temperature sensitivity of the devices. Suggestions are made for further research into the mechanism of the solid contact and into fabrication methods for the devices. It is expected that the method of construction developed here will be applicable to this work.