The bimetallic or trimetallic nanoparticles with controllable sizes and compositions (M1(n)M2(100-n), and M1(n)M2(m)M3(100-n-m)) were synthesized by molecularly-engineered reduction and decomposition reactions in solutions. The resulting nanoparticles were supported on various powder supports including carbon, silica, titanium oxide, and calcined by controlled thermal treatments. The catalysts were examined in catalytic and electrocatalytic reactions, including methanol oxidation reaction and oxygen reduction reaction. The activity and stability of the electrocatalysts in fuel cell reactions were studied by electrochemical techniques. Results were discussed in terms of the synergistic properties of the bimetallic/trimetallic electrocatalysts for fuel cell and gas-phase reactions, providing the fundamental insights into the correlations between the nanocrystal phase and surface binding properties and between the size and composition of the nanoscale bimetallic or trimetallic catalysts. This is an abstract of a paper presented at the 2007 AIChE Annual Meeting (Salt Lake City, UT 11/4-9/2007).