The influence of a variety of parameters on the synthesis of iron oxide nanoparticles (magnetite/maghemite Fe 3 O 4 /Ξ³-Fe 2 O 3 ) by thermal decomposition of a metal-organic iron precursor in an organic medium is reported. We study the role of both the surfactant and the reducing agent on the shape, the size distribution, and the magnetic properties. We aim at synthesizing magnetic nanoparticles with high crystal quality and good magnetic response. A narrow size distribution of pseudospherical and faceted particles (4-20 nm) with a high saturation magnetization (M s β 80-85 emu/g at 5 K) is obtained when using oleic acid as a surfactant. In contrast, decanoic acid yields much larger pseudocubic particles (45 nm) with a wider size distribution and a larger saturation magnetization (M s ) 92 emu/g at 5 K), close to the expected value for bulk magnetite. Besides, the use of a variety of reducing agents monitors the magnetic behavior. In the case of 1,2-hexadecanediol, magnetic characterization suggests that the nanoparticles have uniform oxidation. However, those particles prepared without the use of any reducing agent also show uniform oxidation just with a slightly smaller value of the saturation magnetization (M s ) 76 emu/g at 5 K). In contrast, hydrazine seems to promote a nonuniform oxidation that results in the appearance of the exchange bias phenomenon and in a smaller saturation magnetization (M s ) 67 emu/g at 5 K). New ways to tune the shape, the size, and the magnetic properties are discussed.