The thesis opens with an introduction to some basic electroanalytical principles, and a brief description of carbon-based electrodes, with a particular focus on glassy carbon and carbon fibre microelectrodes. Also included is a review of the most commonly employed electrode modification procedures, with some examples of the analytical applications of modified electrodes. The second chapter describes the use of the bismuth film electrode (BiFE), which consists of a bismuth film electrochemically deposited onto a glassy carbon substrate electrode, in the cathodic electrochemical detection of some reducible organic compounds. The electrochemical behaviour of nitrophenols at BiFE was investigated and the BiFE applied in flow injection analysis amperometric determination of these nitrophenolic compounds. The application of the BiFE in adsorptive stripping voltammetric and potentiometric stripping analysis of cobalt and nickel in some low-volume body fluids is presented in Chapter Three. In addition, the BiFE was successfully applied in the determination of some selected trace heavy metals in soil extract samples, and the results obtained compared with those from ICP-MS measurements. In Chapter Four, the development of a microsensor based on a nickel oxide,ruthenium hexacyanoferrate (NiO,RuHCF) inorganic layer, for the selective detection of ascorbic acid in acidic media is described. The modified microsensor is applied in measurement of ascorbic acid in model gastric juice solution. In addition, the modified microsensor, with a protective cellulose acetate membrane, is employed in the determination of ascorbic acid in real gastric juice solution. The final chapter presents some possibilities for furthering the study, use and application of the modified electrodes used in the work.