The redox polymers [Os(bpy)2(PVP)i0Cl]Cl and [Ru(bpy)2(PVP)10Cl]Cl uncross-linked and cross-linked 10% with 1,10-dibromodecane were used to modify glassy carbon electrodes. Their amperometric surface characteristics in conventional three electrode assemblies were compared with those found in thin layer flow cells of flow injection systems with the working electrode in each case modified with the osmium and ruthenium polymers. The peak-to-peak separation for the osmium polymer increased to 50 mV in the thin layer flow cell compared with 0 mV in the conventional cell. For the ruthenium polymer separation of 60 mV was found compared to 0 mV. Other amperometric surface characteristics were similarly affected. Examination o f the Nemst behaviour was also affected by the changeover from the conventional to the thin layer cell and it was shown that the slope of the Nemst plot decreased from 57 decade'1 to 48 decade'1 for the osmium polymer and from 58 decade'1 to 50 decade'1 for the ruthenium polymer. Analysis of the effect of cross-linking on the stability of these metallo-polymers showed that the time taken for the response to reach half of its initial value, increased from 45 hours, for the uncross-linked osmium polymer to 100 hours for the cross-linked version. For the ruthenium polymer, cross-linking did not affect stability as the time taken for the response to reach half of its initial value was in each case measured at 8 hours. The cross-linked [Os(bpy)2(PVP)10Cl]Cl was shown to mediate the reduction of nitrite in a through layer reaction. The use o f this modified electrode in a thin layer flow cell at 0.12 V vs. SCE in a flow injection manifold, which incorporated a cadmium reduction column, allowed the formation of a nitrate sensor. The sensitivity of this sensor was 0.05 ^A mg’1 cm3 and its linear range was 0.1 to 190 mg I'1 NO3 -N with correlation coefficients > 0.999. The limit of detection was found to be 50 ^g I'1 NO3 -N (S/N=2). It was successfully used for measurement of nitrate in well and river water samples. This sensor was then applied to the determination of NOx by adapting an off-line sampling technique for this gas. A sampling time of 2 hours in a stirred solution o f dilute H2S 0 4 containing H20 2 was found to be sufficient to collect the gas sample. Good correlation was obtained between the measurements of the absorbed gas samples made using this sensor and that obtained with an ion chromatographic sensor. Reduction of free chlorine at the cross-linked [Os(bpy)2(PVP)i0Cl]Cl modified electrode was found to be mediated by a surface reaction at the polymer-solution interface. A thin layer flow cell modified with this polymer was used in a flow injection manifold to develop a free chlorine sensor at 0.1 V vs. SCE. The linear range was calibrated between 0.1 and 50 ppm free chlorine and a limit of detection of 0.05 ppm was found. This modified electrode remained stable for 24 hours. The oxidation of ascorbic acid was also found to be mediated by [Os(bpy)2(PVP)10Cl]Cl by a surface reaction. A thin layer flow cell modified with this polymer was used in a flow injection manifold to develop an ascorbic acid sensor. The working potential was 0.25 V vs. SCE. The sensitivity of this sensor was found to be 7.3 fiA dm'3 mM'1 and the limit of detection was 1 x 10-6 mol dm'3. The sensor was applied to the analysis of ascorbic acid in real samples and the results obtained compared well with a standard titrimetric method.