The formation of spinel iron-oxide nanoparticles during the co-precipitation of Fe3+/Fe2+ ions from an aqueous solution in the presence of carboxymethyldextrane (CMD) was studied. To follow the formation of the nanoparticles, a mixture of the Fe ions, CMD and ammonia was heated to different temperatures, while the samples were taken, quenched in liquid nitrogen, freeze-dried and characterized using transmission electron microscopy (TEM), X-ray diffractometry (XRD) and magnetometry. The CMD plays a role in the reactions of the Fe ions' precipitation by partially immobilizing the Fe3+ ions into a complex. At room temperature, the amorphous material is precipitated. Then, above approximately 30 degrees C, the spinel nanoparticles form inside the amorphous matrix, and at approximately 40 degrees C the matrix decomposes into the suspension of carboxymethyl-dextrane-coated iron-oxide nanoparticles. The CMD bonded to the nanoparticles' surfaces hinders the mass transport and thus prevents their growth. (C) 2015 Elsevier B.V. All rights reserved.