ABSTRACT The Reverse Osmosis desalination has become a technological option to guarantee an adequate drinking water supply in zones with water scarcity. Nevertheless, the process is accompanied by potential adverse impacts on the coastal ecosystem, mainly due to chemical discharges. Taking into account the environmental risk presented by these chemical mixtures, in this work we used a short-term toxicity bioassay with Artemia franciscana nauplii expressed in lethality to exposures individually and their combinations. Results showed that toxicity degree from single exposures was C5H8O2 > NaClO > CuSO4 > KMnO4 > FeCl3, indicated that C5H8O2 was the most toxic biocide. However, all compounds in combination with C5H8O2 exhibited a very strong antagonism. Except, in mixtures, NaClO/FeCl3 and CuSO4/KMnO4 resulted in an additive effect. This environmental assessment will allow reducing risk significantly on the concentrate considering the sensitivity of the marine ecosystem to the application of chemical agents during the desalination process.