My PhD programme of research at the National Centre for Sensor Research has focussed on the development of colorimetric sensors based on pH indicator dye chemistry. Part of the research programme was funded by the Marine Institute of Ireland and Bord Iascaigh Mhara (the Irish Sea Fisheries Board). This was specifically to develop a colorimetric headspace gas sensor to monitor the release of volatile spoilage compounds from fish samples. In our approach, the sensors reacted with target spoilage compounds to produce a measurable change in colour. Novel spectral and colour processing techniques were used to quantitatively monitor changes in the sensor response. The sensor formulation developed was optimised and the sensor response was characterised and calibrated to headspace ammonia. An experimental setup was devised for the routine screening of different fish species in parallel. The microbial populations of fish samples during spoilage were determined, and the working sensor correlated to microbial populations of fish samples tested and to the concentration of headspace spoilage compounds released during the course of a spoilage experiment. In the final section of work, a novel handheld colour scanner device was developed to enable routine measurements of colour changes to be made “on-package”. The device uses 2 LEDs for illumination and a photodiode for detection, and is a low cost, portable device that can be battery-operated. The combination of scanner and sensor was tested with real fish samples, and the results obtained show that the system is operating satisfactorily.