Our group has previously reported the successful development of a steady state microelectrode sensor for the determination of dissolved oxygen concentrations in sea water. The work presented here underpins the development of a fast oxygen sensor for oceanographic applications; the overall aim is to determine the best conditions to measure dissolved oxygen concentrations with microdiscs on a time scale ranging from sub milliseconds to seconds. Previous studies with rotating disc and microelectrodes have shown how the apparent number of electrons, napp, varies between 4 and 2 as the steady mass transfer coefficient increases. The present study also aims to provide insight into the oxygen reduction reaction (ORR) and in particular to probe whether and how napp, depends on the time scale of the reaction. This thesis will describe the results of transient amperometric experiments recorded with microdisc electrodes. The experiments were carried out with different size Pt microdisc electrodes, using fast scan cyclic voltammetry, chronoamperometry and sampled current voltammetry. Model experiments were first conducted with ferrocene, FeCp2, in acetonitrile to validate the experimental approach. Subsequent model experiments were conducted with hexaammineruthenium (III) chloride, Ru(NH3)6Cl3, in aqueous chloride solutions. The ORR was also probed in aqueous chloride solutions. Experiments were also systematically conducted in absence of redox couple to investigate the role of background processes in determining the overall amperometric response over the different time scales considered. Where possible, fast scan voltammograms, chronoamperograms and sampled current voltammograms were compared to theoretical expressions or simulations. The work will also describe attempts to develop a potentiostatic conditioning waveform capable of pre treating the microdisc electrode in order to produce reliable oxygen reduction chronoamperograms. Various coatings were used for simultaneous determination of dopamine and oxygen species for biological application. The conditioning waveform at bare electrode was found to give more reproducible ORR amperometric response than the coatings alone.