Nanoparticle suspensions in liquids have shown remarkable changes in thenno-physical and optical properties of their base media. In this study, carbon nanotubes are chemically functionalized in order to overcome their inherent hydrophobic nature and make them dispersible in a polar liquid such as water. Dispersions of pristine and functionalized carbon nanotube nanofluids in deionized water were prepared and their optical properties at room and elevated temperatures were measured. Quantitative analysis using absorption spectroscopy demonstrates higher radiation absorption of carbon nanotube dispersions with respect to pure water which is decreased by increasing temperature. Optical characterization shows that functionalized carbon nanotube nanofiuids show remarkable stability after undergoing thermal cycling and after extended periods of time (up to three months) in comparison to pristine carbon nanotube nanofluids. (C) 2015 Elsevier Ltd. All rights reserved.