The effect of pH variation of MWCNT-H2O nanofluid on the efficiency of a flat-plate solar collector was investigated experimentally. The experiments were carried out using 0.2 wt.% MWCNT with various pH values, 3.5, 6.5, and 9.5, and Triton X-100 as additive. The procedure of ASHRAE standard was used for testing the thermal performance of flat-plate solar collector. Results show that by increasing or decreasing the pH values with respect to the pH of isoelectric point, the positive effect of nanofluid on the efficiency of solar collector is increased. (C) 2011 Elsevier Ltd. All rights reserved.