Self-assembled monodisperse fcc Co50Ni80 and Co80Ni20 alloy nanoparticles with the average size of 25 and 8 nm respectively are synthesized by reductive thermal decomposition of Co(II)(acac)(2) and Ni(II)(acac)(2) in the presence of surfactants such as oleic acid, oleylamine and trioctylphosphine. The mechanism for formation of colloidal CoNi alloy nanoparticles is explored in this work. The CoNi nanoparticles with high atomic percentage of nickel are found to be more stable and non-interacting. The Co50Ni80 nanoparticles are superparamagnetic at room temperature and exhibit superparamagnetic to ferromagnetic transition at the blocking temperature (T-b) similar to 130-160 K, whereas Co80Ni20 nanoparticles are ferromagnetic at room temperature with low coercivity (similar to 20 Oe). The magnetization value of Co80Ni20 nanoparticles is found to be high as compared to Co50Ni50 nanoparticles due to high atomic percentage of cobalt. Interestingly, size of CoNi alloy nanoparticles with high nickel content is found to be large, which indicates that nickel nuclei act as catalysts for the growth of CoNi alloy nanoparticles in the reaction. (C) 2010 Elsevier Inc. All rights reserved.