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Iron oxide nanoparticles (IONs) were synthesized from ferrous aqueous solutions in air atmosphere by co-precipitation. The effect of [Fe+2]/[Fe+3] ratio on the properties of the nanoparticles were investigated. X-ray diffraction (XRD) patterns showed the charac- teristic peaks of iron oxide for all samples. Fourier transform infrared spectroscopy analysis also confirmed the iron oxide formation. According to magnetic measurements, saturation magnetization, Ms of the nanoparticles increased from 37.6 emu/g to 59.4 emu/g with the increase of [Fe+2]/[Fe+3] ratio from 1/2 to 6/6 and the samples are superparamagnetic with zero coercivities. However, with the increase of the ratio above 6/6 the samples start to show coercivities (8 Oe, 22 Oe and 33 Oe) and Ms increases up to 74.3 emu/g. Particle sizes calculated from the XRD patterns, the images of the transmission electron microscope and magnetic data are consistent with each other and increased with the increase of [Fe+2]/[Fe+3] ratio. It is revealed that the particle size, Ms and magnetic character of IONs were significantly influenced by [Fe+2]/[Fe+3] ratio. Below the critical size of ~11 nm it is seen that IONs showed superparamagnetic character with zero coercivity.