Knowledge of nucleation and growth mechanisms is an important prerequisite for size control during nanoparticle synthesis. However, for many systems the mechanism is not completely understood. Here, investigation of the formation of NixPt1-x nanoparticles that are synthesized via reduction of platinum (II) acetylacetonate and nickel (II) acetate in the presence of oleic acid, oleylamine, and 1,2-hexadecanediol as the reducing agent is reported. Different reaction conditions are studied to reveal the influence on particle size and composition. The composition of the NixPt1-x nanocrystals is tunable through variation of the Ni:Pt ratio. The particle size depends on the concentration of oleic acid, while oleylamine mainly determines the particle shape. The binding of both stabilizing agents on the nanoparticle surface is investigated by attenuated total reflectance Fourier transform infrared spectroscopy. The growth process of the particles is studied by transmission electron microscopy analysis and UV-vis spectroscopy at different reaction stages. Our results lead to the assumption of a single nucleation event, followed by the growth process in the absence of Ostwald ripening. This type of growth can be extended by continuous injection of the molecular precursors, which allows the possibility of particle size control.