Ferrimagnetic iron oxides are the common choice for many current technologies, especially those with application in biology and medicine. Despite the comprehensive knowledge accumulated about their chemistry in the bulk state, the sequence of changes taking place during the precipitation of iron oxide nanoparticles in aqueous media is much less extensive. We show that using sodium carbonate as a co-precipitating agent for the synthesis of uncoated iron oxide nanoparticles, the reaction proceeds sufficiently slowly to enable a detailed study of both the reaction pathway and products. The effect of pH, temperature and reaction time on particle size, morphology, crystalline phase and its magnetic properties was investigated. The obtained nanoparticles showed an increase in average particle size of about 10 nm per pH unit for the magnetite phase leading to 6.9 ± 0.4 nm, 18 ± 3 nm and 28 ± 5 nm for pH 8, 9 and 10 respectively. Goethite was initially formed by an olation mechanism at room temperature, followed by a slow transformation into magnetite over a 24 h period, as tracked by X-ray diffraction. In another set of experiments where the reaction temperatures were varied, magnetite was obtained directly by the oxolation mechanism at temperatures above 45 °C. The optimization of the experimental parameters led to superparamagnetic nanoparticles with a high saturation magnetization of 82 A m2 kg−1 at 300 K when synthesized at pH 9.