Pt-nanoparticles with tuneable size between 0.8 nm and 8 nm supported on nanocrystalline CeO2 was prepared by surfactant promoted hydrothermal synthesis method and their activity was studied for low temperature water-gas-shift (WGS) reaction. The catalyst was characterized by XRD, ICP-AES, N-2-sorption, H-2-pulse chemisorption, TPR, XPS, SEM, TEM, and EXAFS. The average Pt particle size, measured by HR-TEM were 0.8 nm, 3.7 nm and 7.8 nm when the Pt loading on CeO2 was 0.9 wt%, 1.9 wt% and 4.7 wt%. The catalyst with 0.9 wt% Pt loading supported on CeO2 (0.9Pt-CeO2) showed the best catalytic activity of 85.1% CO conversion with CO2 formation rate of 1.66 X 10(10) mol cm(-1) s(-1). The activity of the catalysts decreased with increasing Pt size and Pt loading on CeO2. The influence of reaction parameters like time-on-stream, temperature, gas hourly space velocity (GHSV), etc. were also studied in detail. The XPS study revealed that the fresh catalyst contained Pt2+, which is the active species for water-gas-shift reaction. The catalyst (0.9Pt-CeO2) did not change any activity till 30 h in time-on-steam at 140 degrees C showing the stability of the Pt-nanoparticles at 140 degrees C. When the size of the Pt was increased above 1 nm the WGS activity decreased with time as the Pt2+ reduced to metallic Pt at 140 degrees C. (C) 2014 Elsevier B.V. All rights reserved.