We herein demonstrate an in-situ device and a real time method to observe the vital stages of Fe-oxide nanoparticles' (NPs) preparation by coprecipitation reaction. The reaction volume is monitored by a home-made AC susceptometer to analyze the magnetic response of the growing NPs. Three different regions (nucleation, growth and diffusion) could be identified in the recorded in-phase and out-of phase curves. The morphological and magnetic characterization of the obtained Fe-oxide NPs reveal minute particle dimensions ( < 3 nm), narrow polidispersity (1-7 nm), superparamagnetic behavior (null coercivity) and narrow distribution of blocking temperatures ( < T-B > similar to 67 K). We show that the device and technique enable real time assessment of the mixing speed which has paramount impact on the physical and chemical properties of the final product. (C) 2015 Elsevier Ltd. All rights reserved.