In this study the development of an improved electrochemical fuel cell sensor for the detection of low-levels of formaldehyde vapour is described. Current fuel cell sensors used in the commercial Formaldemeter™ instrument lack sensitivity, respond to methanol, and show an interference response to humidity. For the detection and accurate determination of low-level concentrations of formaldehyde an improvement in these characteristics is essential. Two routes were investigated to achieve these requirements. Firstly the oxygen reduction reaction at the cathode was replaced with that of a conducting polymer, polyaniline (P ANI). Before construction of the fuel cells the stability and open circuit behaviour of various P ANI films were investigated in order to assay their suitability as a cathode material in a fuel cell application. Based on these results various fuel cells containing P ANI as the cathode were prepared and incorporated into the FormaldemeterTM. The cells were then evaluated using a calibrated vapour stream. Fuel cells containing a Pt-black anode and a P ANI/SOlcoated Pt-black cathode in H2S04 electrolyte displayed a significant improvement in sensitivity, selectivity and a reduced humidity response. In addition a fast response time, excellent reproducibility and a long shelf life was demonstrated. It is proposed that a P ANI film coated on Pt-black behaves as a more stable electron sink for the cathodic reaction instead of the oxygen reduction reaction which is dependent on the diffusion of oxygen. The fast fuel cell response time is explained by the fast switching of PAN I « lOJls). In the second part of the work, the effect of changing fuel cell catalyst was investigated. Initially, the electrocatalytic activity of various noble metals and alloys electrodeposited on glassy carbon were investigated. Based on these results several fuel cell electrodes were prepared and characterised by SEM and EDAX. Fuel cells were then constructed and their performance evaluated. Cells made up of 50:50 % atomic weight ratio of platinum and palladium showed that while the fuel cell was active towards low levels of formaldehyde, its response to equivalent levels of methanol was very small.