This paper experimentally investigates the extinction coefficients of water-based multi-walled carbon nanotubes (MWCNT) nanofluids for application in direct-absorption solar collectors (DASC). As a solar thermal energy absorbing medium, water-based MWCNT nanofluids with low volume fraction are prepared and their extinction coefficients are initially measured using Lambert-Beer law at a fixed wavelength (632.8 nm). The results show that the incident solar energy can be completely absorbed in the penetration depth of 10 cm using the nanofluids with extremely low volume fraction of 0.0005 Vol.%. The study demonstrates that the extinction coefficients of water-based MWCNT nanofluids increase linearly with the volume concentration of MWCNT in the range from 0.0005 to 0.005 Vol.%. The extinction coefficients including present and previous data are compared with the theoretical models which are the Maxwell-Garnett for cylinder and the Rayleigh scattering approximation. Based on the results, it is clearly shown that the conventional Maxwell-Garnett model which can consider the shape effect is inaccurate for predicting the extinction coefficient of nanofluids. However the Rayleigh scattering approximation which can consider size effect can qualitatively predict the extinction coefficient of water-based MWCNT nanofluids. (C) 2013 Elsevier Ltd. All rights reserved.