The systematic study of the synthesis of nanocrystalline cobalt in trimethylene glycol (TMEG) is presented. The solution chemistry affected the kinetics of the metalforming reaction and hence, the stability of cobalt phases. The addition of OH- ions into the cobalt solution in TMEG not only accelerated the formation of the magnetic phase but also affected the stability of the metallic precipitate. Depending on the OH-/Co mole ratio and the type of cobalt salt, hep, fee and metastable pseudo-cubic ε-Co phases were formed. Highly monodisperse ε-Co nanoparticles were produced when cobalt acetyl-acetonate salt was used instead of the acetate one. Furthermore, the presence of Pt ions in starting solutions results in a dramatic shortening of the reaction time. The saturation magnetization (Ms) and coercivity (Hc) values of the cobalt samples ranged from 80 to 110 emu/g and 101 to 211 Oe, respectively. The higher coercivity of 248 Oe was found when CoPt co-existed in the precipitates produced at 214°C.