This thesis describes the development and application of hydrodynamic modulation voltammetric (HMV) techniques coupled to ultramicroelectrodes (UMEs) that possess intrinsically high mass transport rates in quiescent solutions. This study demonstrates that the well defined convective-diffusive conditions of the microjet electrode (MJE) arrangement allows mass transport to be enhanced by almost two orders of magnitude compared to a 25 mm diameter disc-shaped UME. The MJE comprises a nozzle which is used to deliver solution to a UME surface at high velocity. Scanning electrochemical microscopy (SECM) with small UMEs has been used to image the hydrodynamics of the jet system with high precision. Variations in local mass transport for both IrCl63- and Fe(CN)64- oxidation at a range of flow rates has been observed at various positions within the impinging jet and the stagnation zone has been thoroughly characterised under a variety of experimental conditions. Agreement has been found between experiment and theory for voltammetric data recorded with the nozzle and UME aligned in the stagnation zone, for a range of viscous solutions examined. By modulating the mass transport rate to the surface of an UME, in the MJE arrangement, by the introduction of a rotating blade between the end of the nozzle and the UME, it was possible to enhance the current sensitivity of the system. Trace level detection, to 2 x 10-7 mol dm-3 IrCl63- solution, was readily facilitated. This type of HMV experiment has utilised two methods to provide the reference signal for phase-sensitive detection of the current signal, involving either a dual-disc electrode or a single UME coupled to an LED detection system. Both HMV methods have been shown to work well.