The Microfluidic Analyser for pH and Chemical Oxygen Demand (MApCOD) project is the latest stage in the development of a microfluidic platform for autonomous monitoring of environmental water quality. pH measures the acidity or basicity of water. Most aquatic animals are adapted to a range of 6.5–8.0 (close to neutral, pH 7.0). Low pH can also allow toxic substances such as ammonia to become more available for uptake by aquatic plants and animals, greatly increasing their effective toxicity. pH is commonly measured in industrial and municipal wastewaters as well as in monitoring of drinking water, of surface waters such as rivers and lakes, and in many industrial processes. In this project pH is measured using a combination of pH indicators, optimised to give a colorimetric response over the pH range 4.0–10.0, which covers the range of pH values commonly encountered in monitoring of surface waters, drinking water and many wastewaters. Dual LEDs and a photodiode are used to measure light absorbance at appropriate wavelengths (430 and 570 nm). The responses of the two pH indicators are complementary, allowing a colorimetric response to be obtained over the pH range of interest.