The development of LED based instrumentation for monitoring water quality is discussed. Many of the standard colorimetric tests for water quality monitoring coincide spectrally with widely available LED sources. Existing monitors use incandescent filament-based sources combined with narrow-band interference filters. Such systems suffer from problems of stability, bulk, cost and degradation with time. The replacement of such sources and filters with LED’s overcomes many of these problems. The initial experimental work involved identification of suitable LED’s for detection of various analytes in water. In particular, LED’s were selected on the basis of spectral match to standard colorimetric tests for species such as Fe, Al, Mg and P. Tests were carried out on a specially constructed bench-top system. Various path length cells were tested and based on these results, LED’s were incorporated into existing on-line devices, replacing bulbs and filters. This involved the construction of the necessary electronics and changes to the resident software and mechanical design of the device. The final part of the work involved construction of a prototype device capable of measuring concentrations of iron, aluminium, manganese and phosphate in drinking water as well as colour and turbidity. The device is portable, incorporates a full reagent mixing and delivery system and has limits of detection below EC directives for all the species listed above. This system was fully characterised in terms of limits of detection, linear ranges, accuracy, stability and repeatability.