Synthesis of nanoparticles with particle size control is provided by the method of using two different metal-containing precursors, a capping component, an optional reducing agent, and then contacting the two precursors with the capping component to form a reaction solution, which is heated to produce first and second metals-containing nanoparticles. By controlling the ratio of the concentration of the capping component to the total concentration of the two metal-containing precursors, the nanoparticles can have diameters ranging between about 1 nm to about 15 nm. A decrease in the concentration of the capping component typically increases the size of the nanoparticles. Preferred compositions include Pt and Co-containing alloy nanoparticles. Controlled synthesis of larger, about 6 nm to about 12 nm, sized nanoparticles can be achieved in a solvent-free reaction process.