There are several approaches available for the synthesis of nanoparticles below 20 nm, which is also the critical size for superparamagnetic iron oxide nanoparticles (SPIONs). In this study, we aimed to determine optimum synthesis conditions for SPIONs via a co-precipitation method as well as demonstrate the effect of each parameter on the size, morphology and magnetic character of each nanoparticle synthesized. The effects of Fe+2/Fe+3 ratio, pH and ionic strength on the morphology, size, size distribution and saturation magnetization (Ms) value of SPIONs were investigated systematically. In addition to this, the importance of using inert atmosphere was investigated and attempts to discover alternative approaches involving synthesis under air atmospheric conditions were made. Transmission electron microscopy (tEM) images show that approximately 5-6 nm sized SPIONs were synthesized successfully under both air and N-2 atmosphere. By adjusting the pH and the ionic strength, the growth mechanisms of particles were changed. Crystallinity of the particles was examined with an X-ray diffraction (XRD) method and the characteristic peaks of Fe3O4 nanoparticles were also obtained. Vibrating Sample Magnetometer (VSM) measurements revealed that Ms increases with increasing ionic strength. Furthermore, it was found that the geometry of spherical nanoparticles can be changed to nanoflakes by-tuning the Fe+2/Fe+3 ratio. This study demonstrates, comparatively, the effects of the changes of co-precipitation parameters on the various properties of SPIONs. (C) 2016 Elsevier B.V. All rights reserved.