The development of a miniaturised microfluidic instrument for monitoring phosphorus in natural waters from the optimisation of the chemistry through to the fabrication of the microfluidic manifold in polymeric materials is presented. The research initially was concerned with optimising the yellow colorimetric method for a phosphate determination and its transferral to a Si-etched microfluidic chip configuration. Th is simple method employs one reagent mixed in a 1:1 ratio with an orthophosphate-containing sample to produce a yellow colour absorbing strongly below 400nm. A stopped flow approach is used which, together with the very rapid kinetics and simple reagent stream, enables a very uncomplicated microfluidic manifold design to be adopted. The working wavelength wa s 380nm, which coincided with the peak output of a recently developed U V -L E D narrow bandwidth light source. The limit of detection for the yellow method is 0.2 mgL'1 P O ^ - P with a linear range from 0 - 5 0 mgL*1 P O 43“ - P possible. T h e reaction time at room temperature is less than 3 minutes, which m ean s up to 20 sam ple s / hour can be analysed. The next stage in the research involved applying the results obtained in the Sietched microfluidic chips to the design and fabrication of a microfluidic manifold in polymer materials. Chips were made by a combination of microfabrication techniques including a C 0 2 laser ablation, hot embossing and micromilling. Transferring the technology to a polymeric platform required a whole new set of experiments to be undertaken. The key is su e s add ressed were multiple layer alignment, optical detection, bonding of polymeric materials; the provision of leakfree fluidic interconnects to external tubing and reproducible analytical measurements.