The main aim of this work was to develop an SO2 sensor based upon a conducting polymer. The methodology and properties of a conducting polymer sensor along with the electrochemical characterisation of various polypyrrole layers is described. It was found that polypyrrole layers containing Copper (11) Phthalocyanine - 3,4',4", 4"'-tetrasulphonate anions (PPTSP), as the dopant, upon electrochemical switching exhibit cation movement in and out of the polymer matrix. A concentration, pH, temperature and scan rate study was performed on the process of cation movement. Also anions, such as CIO4 ", dodecylbenzenesulphonate (DBS) were employed as the dopants within the polypyrrole matrix. In this work it is described that the low level of non-faradaic current, characteristic of PPDBS layers, was due to the presence of the DBS anion in solution rather than the polymer morphology. Differential Pulse Voltammetry is applied to various polypyrrole layers and sigmoidal differential current outputs were obtained. A model is proposed for this behaviour which includes a set of resistances in parallel. Potential Step experiments were also performed on the polypyrrole layers. A transmission line model is proposed to explain the behaviour of the polymer film. For several electroconducting polymers spectroelectrochemical studies were carried out. The absorption spectra are quantitatively analysed using the Nemst equation by a 'Monomer Unit Model. The reaction between pyrrole and tetracyanoethylene (TCNE) was studied electrochemically and spectroscopically. It was found that TCNE forms a charge transfer complex with pyrrole, and it is present within the PPTCNE layer as the dopant anion. The influence of solvent composition upon the ferricyanide/ferrocyanide system was investigated electrochemically. It was found that in organic solvents ferrocyanide is a strong reducing agent. Polypyrrole layers were deposited across interdigital array electrodes and their responses to various organic vapours and 5 % SO2 were investigated. The use of Prussian Blue films as possible overlayers upon the polypyrrole layers is described. Also solid state voltammetry was performed upon Prussian Blue films, and when these mixed valent films were exposed to certain saturated vapours in nitrogen, good reversible responses were obtained. Also the interaction of organic vapours and polypyrrole films, deposited upon ITO electrodes, was investigated by UV/vis spectroscopy.