A copolymer incorporating polyaniline was used as a sensing medium in the construction of a resistance based humidity sensor. Aniline monomer was polymerised in the presence of poly (butyl acrylate / vinyl acetate) and a copolymer containing polyaniline emeraldine salt was obtained. The sensing medium was then developed by redissolving 1-2 w/w% of the resulting polymer residue in dichloromethane to produce a processable polymer blend solution. Some of this polymer residue was also de-doped in a solution of ammonia, and then washed with distilled water until the waste water had a neutral pH. This residue was then redissolved at 1-2 w/w% in dichloromethane to produce a second processable polymer blend this time containing polyaniline emeraldine base. The final sensor design utilised 125μm polyester insulated platinum wire as conducting electrodes that were dip coated in the emeraldine salt copolymer solution and allowed to dry in a desiccator. The sensor was then dip-coated in a protective barrier layer of the emeraldine base copolymer to prevent over-oxidation and/or de-protonation of the emeraldine salt sensing medium under this coating. The sensors had an overall final thickness of less than 150μm and showed high sensitivity to humidity, low resistance, and good reversibility without hysteresis. Sensors were monitored for 2-probe resistance changes when in contact with water. Calibration curves for each sensor were produced to convert the resistance reading to mass uptake of water. Individual sensors were embedded within Aluminium 5083 / Araldite 2015 adhesive joints to monitor mass uptake of water when exposed to marine environments. Correlations between mass uptake of water and joint strength were made. There are various advantages of such a sensor design. Polymer based thin film humidity sensors have the advantage that the high processability of the material allows for simple fabrication of a range of geometries including smaller sensor designs. The ease of processing gives a low cost sensor, whilst the small size and good mechanical properties gives a robust sensor which has the flexibility to be able to be used in applications where dynamic stresses and strains are encountered. Such sensors may find uses in a number of areas including electronic textiles, food/ electronics packaging and corrosion detection.