The use of osmium- and ruthenium-containing redox polymer modified electrodes for the development of novel electrochemical sensors has been investigated. The kinetics and transport parameters for the electrocatalytic reactions of a number of important substrates at the modified electrodes have been analysed using conventional theory for mediated electrocatalysis. Using this information, a senes of novel sensors have been constructed, and the operational performance of these devices assessed. A sensor for the reductive detection of nitrite was constructed from an [Os(bipy)2(PVP)ioCl]Cl modified electrode. The mechanism of nitrite reduction at the modified electrode appears to proceed through the nitrosomum ion The cross-exchange reaction occurs throughout the polymer film and is controlled by the rate constant for the cross-exchange reaction Slow reaction kinetics allows measurement of limiting currents independent of solution mass transport. Under these conditions, an equation relating sensor response directly to known parameters can be obtained, this equation can be used for the internal calibration of the sensor. The sensor was applied to the determination of mtnte in saliva. A novel dual sensor for the speciation analysis of redox couple Fe(II)/Fe(III) was constructed from osmium- and ruthenium-containing redox polymer modified electrodes. The principle of operation is described, and is based on the free energy differences for both the oxidation and reduction of the analytes at their respective sensors. The detection system exhibits considerable advantages compared to existing speciation techniques. The operational performance of the dual sensor is discussed in relation to existing methods for this speciation analysis The recoveries of Fe(II) and Fe(III) from water samples was assessed. The physical stabilisation of the redox polymers by chemical cross-linking and by manipulation of the polymer back-bone was attempted. The effects of the stabilisation procedures on the charge / mass transport and electrocatalytic properties of the modified electrodes was investigated, and are discussed in relation to the requirements of operational sensors. Finally, the performance of sensors constructed from redox polymer modified electrodes is discussed with respect to control of sensor sensitivity, selectivity and potential interferences General strategies for the control and optimisation of sensor performance are also discussed.