The determination chromium speciation in the field is a significant analytical challenge. While chromium exists in oxidation states from 0 to VI, it is predominantly found in the (III) and (VI) states [1]. Industry effluent (e.g. textile/electroplating) is a common source of chromium pollution in the environment. Due to corrosion inhibitors used in pipes, and contamination leaching from sanitary landfills, drinking water supplies can become contaminated also [2]. The bioavailability and toxicity of chromium is largely dependent the oxidation state of the element [2]. Consumption of Cr (III) is an essential component in human diet, as it is responsible for maintaining glucose, lipid and protein metabolism [3]. In contrast, Cr (VI) is strongly oxidizing, exhibiting high toxicity, with carcinogenic and mutagenic properties [4]. It is recommended by the World Health Organisation (WHO) that the maximum allowable concentration of chromium (VI) in drinking water is 0.05 mg L−1 [5]. Handheld colourimeters for on-site measurements are a convenient option for frequent water monitoring; however the limit of detection (LOD) of these devices is typically higher than the recommended limit. Microfluidic ‘lab-on-a-disc’ technologies were used in the development of an optical sensor for chromium speciation in water. The principal behind these devices is to minimize laboratory processes onto a microfluidic system that can be brought to the sampling site for rapid sample-to-answer analyses. The objective for this device was to design and fabricate a fully integrated optical sensor for on-site measurement of both trivalent and hexavalent chromium in freshwater. A strong focus was placed on maximizing sensitivity in order to achieve a low LOD.