Abiotic and biotic processes catalyzed by ferric (hydr)oxide nanoparticles (NPs) in aquatic media underlie many important nanotechnologies including chemical catalysis, biomedical engineering, remediation of contaminants, and clean renewable energy sources. These processes have been extensively addressed in the past and commonly interpreted within the so-called chemical paradigm neglecting semiconducting properties of the NPs. Using batch kinetic measurements coupled with X-ray photoelectron spectroscopy (XPS) as well as a new method that combines in situ FTIR spectroscopy and ex situ XPS, we study the effect of size on the catalytic oxygenation of Mn(II) in the presence of hematite (alpha-Fe(2)O(3)) and ferrihydrite NPs. We find that oxidative catalytic performance of these NPs degrades with decreasing their size. Using Density Functional Theory (DFT) modeling, we show that this unusual trend cannot be rationalized within the traditional chemical paradigm. Given the nanosize-induced changes in the electronic properties of ferric (hydr)oxides and the thermodynamic properties of the system, we demonstrate that the catalytic reaction proceeds through the electrochemical pathway. The new mechanistic insights suggest new interpretation of literature data, open up new possibilities for manipulating technological performance of metal oxides, and conceptually broaden our general understanding of (bio)chemical activity of NPs. (C) 2011 Elsevier Inc. All rights reserved.