My PhD programme of research at the National Centre of Sensor Research has focussed on the development of a miniaturized detection system based on the concept of a Paired Emitter-Detector Diode (PEDD) LED optical sensor as a generic optical sensor platform for colorimetric analysis as developed within the research group. This research was funded by Science Foundation Ireland (SFI). This novel optical sensor employs two LEDs, operating one as a light source and the other as a light detector. The emitter LED is forward biased and the detector reverse biased. A simple timer circuit measures the time taken for the photocurrent generated by the emitter LED to discharge the detector LED from 5 V (logic I) to 1.7 V (logic 0). PEDDs of various wavelengths and designs were investigated and initially calibrated using pH indicator dyes. The PEDD has been applied as an inexpensive detector in a flow-injection system for determining low concentration levels of phosphate employing the malachite green spectrophotometric method. The novel flow detector employed within this manifold is a highly sensitive, low cost, miniaturized light emitting diode (LED) based flow detector, which achieved an improved LOD in comparison to a LED-photdiode sensor. The PEDD has also been applied as a photometric detector in HPLC. Separation of transition metal ions, manganese (11) and cobalt (11) were carried out using a Nucleosil 100-7 (functionalised with IDA groups) column. The PEDD was calibrated using Mn (11) and Co (11) PAR complexes providing a linear response. Higher sensitivity and improved precision were obtained from the PEDD compared to the commercially available UV-vis variable wavelength detector.