The platinum–gold alloyed nanoparticles were prepared via decomposition of double complex salts containing both inorganic and organic ligands. Decomposition of the precursor was performed in a reductive atmosphere with varied temperature ramping rates. The samples were characterized by means of X-ray diffraction (XRD) analysis, transmission electron microscopy, and UV–vis spectroscopy. According to XRD data, both the ramping rate and the final temperature of decomposition strongly affect the Pt–Au ratio in alloyed nanoparticles which are being formed. The average crystallite size was found to be 15–20 nm, that was also confirmed by UV–vis spectroscopy for the alloyed Pt–Au nanoparticles supported on alumina.