Au-Fe, Au-Ni, and Au-Co alloyed nanoparticles electrodeposited on an amorphous carbon electrode from aqueous electrolytes in overpotential and underpotential deposition regions of transition metals (Fe, Ni, CO) were investigated by means of transmission electron microscopy and energy-dispersive X-ray analysis. From selected area electron diffraction patterns of the nanoparticles, it was proved that the Au-Fe, Au-Ni, and Au-Co nanoparticles were homogeneously alloyed, that is, the nanoparticles were alloyed solid solutions, and the composition of the alloyed nanoparticles was dependent on the electrodepositing potential. Multiply twinned particles (MTPs) in these alloyed particles were observed. The MTPs were suggested to be alloyed particles. It is proposed that due to the smaller radii of the deposited transition metal (Fe, Ni, Co) atoms mixed in the Au top-layer of the growing particles there is a stronger tendency for the alloyed particles to grow into larger MTPs that are more stable than Au MTPs of the same size at high negative potentials.