The aim of the work described in this thesis is the characterisation and development of ac impedance biosensors systems through the evaluation of the electrical, physical and chemical processes involved, in order to provide a better understanding of the complex performance of these devices. The investigations of ac impedance biosensors included here cover equivalent c ircuit analysis, the characterisation and optimisation of an ac impedance model urease electrode and the design and proof-of-feasibility of an ac impedance biosensor for the detection of creatinine in urine, important for its medical applications. The activity of urease is assessed in solution and immobilised forms and the dependence of the urea biosensor performance on enzyme loading, immobilisation method, temperature, pH and electrolyte concentration reported. We show that Nafion-immobilised urea sensors offer optimum performance in tests on serum samples, combining high sensitivity and long shelf-life with giving no sign of electrode polarisation effects. High intra-batch reproducibility is also reported. Following the interest of British Nuclear Fuels Ltd. in the detection of creatinine in urine, creatinine bi.osensors constructed with two different enzymatic systems are investigated. A single-enzyme creatinine deiminase electrode and a multi-enzyme creatininase / creatinase / urease electrode are characterised with similar results. Based on our previous work using Nafion and its useful electrical properties at high ionic strengths, this polymer was employed to immobilise the enzymes on interdigitated gold electrodes. The performance of these creatinine biosensors in urine samples is assessed and shown to offer a viable alternative to the standard spectrophotometric creatinine assay. The equivalent circuit modelling of the biosensor system demonstrates that, depending on the immobilisation process, the ionic product generated at the enzyme electrode either diffuses towards the electrolyte or remains trapped in the enzyme phase. Relatively low sensitivity was recorded when the ions diffused towards the bulk phase, and high sensitivity, but low reproducibility was observed when the ions stayed trapped within the enzyme layer.