This work is carried out as part of the COMMON SENSE European FP7 project. The COMMON SENSE project aims to provide a reliable sensing platform for in-situ measurements on key marine water quality parameters relating to eutrophication, heavy metal contaminants, marine litter and underwater noise. The COMMON SENSE nutrient sensor is based on a combination of microfluidic analytical systems, colorimetric reagent chemistry, low-cost LED-based optical detection, and wireless communications. The reliable quantification of nutrients in marine environments is challenging due to the low concentration of these solutes in the ocean and the nature of the matrix in which they are held. Initial studies are focussed on validating a method for the sequential determination of nitrite and nitrate in marine environments. Coupled with the traditional well established Griess–Ilosvay reaction for the determination of nitrite, a vanadium chloride (VCl3) solution is used as the reducing agent. The method shows potential as an alternative to the toxic cadmium column for the reduction of NO3- to NO2- in marine water as results indicate that there are no apparent interferences from variances in salinity. The method was tested on a series of samples with varying salinities and sample matrices (costal, estuarine and freshwater), the method is low cost, reproducible and requires low volumes of sample and reagents.