The aim of the present study is to link the properties of abiotically precipitated crystals with their formation conditions and to investigate their oxygen isotopic signature at different supersaturation states of the solution. We therefore investigate the role of the saturation state in the solution and its effect on (1) the mineralogy of the precipitated crystals, (2) the reaction rates of the precipitation, and (3) the magnetite–water oxygen isotopic fractionation factor. We have thus studied the morphology, size, crystallographic characteristics, and oxygen isotopic compositions of crystals of magnetite of nanometric sizes, made abiotically, using different initial saturation states of the aqueous solution in which they were synthesized at normal conditions of pressure and temperature. The method of synthesis given in Vayssie`res et al. (1998) which enables the formation of magnetite at constant chemical affinity conditions was followed. The inorganically produced magnetite nanoparticles were analyzed by transmission electron microscopy (TEM) to determine their structure and by mass spectrometry for their oxygen isotopic compositions.