The electrocatalytic oxidation of small organic molecules, such as formic acid and methanol, both in acid and alkaline medium, has been the subject of many investigations due to their fundamental interest in the field of Electrocatalysis [1] and Energy Technology (Fuel Cells [2] and Electrolysis [3-4]). Andrzej Wieckowski was one of the first to carry out fundamental studies on the mechanism of the adsorption and oxidation of formic acid and methanol on platinum electrodes using a radiochemical method coupled to electrochemical techniques [5-7]. The use of single crystal electrodes brought a new insight in understanding the role of the catalytic electrode in the adsorption of both formic acid [8-10] and methanol [11-12] on low-index Pt(h,k,l) and Au(h,k,l) electrodes. On the other hand the early development of Infrared Reflectance Spectroscopy by Alan Bewick allowed identifying unambiguously the presence of linearly adsorbed CO as the main adsorbed species resulting from the dissociative chemisorption of HCOOH or CH3OH and blocking the active sites of smooth Pt electrodes [13-14]. As a consequence the adsorption of CO [15] was particularly studied on carbon supported Pt nanoparticles working as catalytic layers in Direct Methanol Fuel Cells (DMFC) [16] or Direct Formic Acid Fuel Cells (DFAFC) [17]. In order to decrease the amount of adsorbed CO resulting from the chemisorption of HCOOH or CH3OH different bimetallic and ternary Pt-based or Pd-based catalysts were developed, particularly for the Direct Oxidation Fuel Cells [18-19]. In this communication we will present several results on the investigation of the reaction mechanism of the electro-oxidation of formic acid and methanol on well-defined smooth electrodes and carbon supported nanocatalysts using combined electrochemical methods and physico-chemical methods (radiochemical technique and Infrared Reflectance spectroscopy). The understanding of reaction mechanisms leads to a better knowledge of the role of the electrode nature and structure allowing conceiving more efficient electrocatalysts. This will be illustrated by some results relevant of energy technology, e.g. the Direct Oxidation Fuel Cell (DMFC, DFAFC) and the Proton Exchange Membrane Electrolysis Cell (PEMEC) to produce clean hydrogen to feed a Proton Exchange Membrane Fuel Cell (PEMFC). References : [1] Catalysis and Electrocatalysis at Nanoparticle Surfaces, A. Wieckowski, E. Savinova and C. Vayenas (Eds.), Marcel Decker Inc., New-York (2002). [2] Electrocatalysis of Direct Methanol Fuel Cells, H. Zhang and H. Liu (Eds.), Wiley-VCH, Weinheim (2009). [3] S. R. Narayanan, W. Chun, B. Jeffries-Nakamura, T. I. Valdez, US Patent 6533919, March 18, 2003. [4] C. Lamy, A. Devadas, M. Simoes, C. Coutanceau, Electrochim. Acta, 60 (2012) 112-120. [5] A. Wieckowski, J. Sobkowski, A. J. Onska, J. Electroanal. Chem., 55 (1974) 383-389. [6] A. Wieckowski, J. Sobkowski, J. Electroanal. Chem., 63 (1975) 365-377. [7] P. Waszczuk, A. Wieckowski, P. Zelenay, S. Gottesfeld, C. Coutanceau, J.-M. Léger, C. Lamy, J. Electroanal. Chem., 511 (2001) 55-64. [8] R.R. Adzic, W.E. O'Grady, S. Srinivasan, Surf. Sci., 94 (1980) L191-L194. [9] J. Clavilier, R. Parsons, R. Durand, J.-M. Léger, C. Lamy, J. Electroanal. Chem., 124 (1981) 321-326. [10] A. Hamelin, C. Lamy, S. Maximovitch, C.R. Acad. Sci., 282 C (1976) 403-406. [11] J. Clavilier, C. Lamy, J-M. Léger, J. Electroanal. Chem., 125 (1981) 249-254. [12] E. Herrero, K. Franaszczuk, A. Wieckowski, J. Phys. Chem., 98 (1994) 5074-5083. [13] B. Beden, A. Bewick, C. Lamy, K. Kunimatsu, J. Electroanal. Chem., 121 (1981) 343-347. [14] B. Beden, A. Bewick, C. Lamy, J. Electroanal. Chem., 150 (1983) 505-511. [15] C. Rice, Y.Y. Tong, E. Oldfield, A. Wieckowski, F. Hahn, F. Gloaguen, J.-M. Léger, C. Lamy, J. Phys. Chem. B, 104 (2000) 5803-5807. [16] S. Park, Y.Y. Tong, A. Wieckowski, M.J. Weaver, Langmuir, 18 (2002) 3233-3240. [17] C. Rice, S. Ha, R.I. Masel, P. Waszczuk, A. Wieckowski, T. Barnard, J. Power Sources, 111 (2002) 83-89. [18] C. Lamy, A. Lima, V. Le Rhun, F. Delime, C. Coutanceau, J.-M. Léger, J. Power Sources, 105 (2002) 283-296. [19] C. Rice, S. Ha, R.I. Masel, A. Wieckowski, J. Power Sources, 115 (2003) 229-235.