A series of x wt.% Pt/CeO2 catalysts (x = 0.1-2.0) was prepared to investigate for the first time the effect of Pt particle size on important kinetic and mechanistic aspects of the water-gas shift (WGS) reaction, namely the concentration (mu mol g(-1)) and chemical structure of active adsorbed reaction intermediates present in the "carbon-path" and "hydrogen-path" of the WGS reaction at 300 degrees C. For this, steady-state isotopic transient kinetic analysis (SSITKA) coupled with in situ DRIFTS and mass spectrometry experiments was performed using D2O and (CO)-C-13. A novel transient isotopic experiment performed allowed to quantify the initial transient rates of reactions of adsorbed formate (-COOH) and CO by water, based on which it was concluded that formate should not be considered as an important intermediate. According to the present work, it is proposed that the WGS reaction on ceria-supported Pt at 300 degrees C occurs largely via the "redox" mechanism, and to a lesser extent via the "associative formate with -OH group regeneration" mechanism. The TOF (s(-1)) of WGS was found to vary only slightly with Pt particle size (1.3-8.0 nm), while the specific reaction rate based on the length of periphery of Pt-CeO2 interface (mu mol cm(-1) s(-1)) was found to significantly increase with increasing platinum loading and mean particle size in the 250-300 degrees C range. (C) 2011 Elsevier Inc. All rights reserved.