The thermal decomposition of acetylacetonate precursors is one of the most employed syntheses to prepare high quality colloidal magnetic nanoparticles. In this paper, an advanced version of this synthetic approach was developed to prepare cobalt ferrite nanoparticles, introducing for the first time a rigorous control on one commonly neglected reaction parameter, that is, the residual oxygen content in the reaction environment. A new concept derived from the statistical analysis of S(T)EM images, i.e., the so-called aspects maps, was introduced: this tool has allowed us to clearly identify the optimal value of pressure to produce particles with an average size ∼19 nm and with a very narrow size distribution (polydispersity 0.4 nm −1). The magnetic properties of this sample were also analyzed, and a strong improvement of the magnetization reversal mechanism, which is a critical issue for several technological applications, was observed. ■ INTRODUCTION Advanced synthesis approaches, necessary to achieve a strict control of the structural, morphological, and chemical properties of nanomaterials, are at the basis of a reproducible manipulation of their unique physical behavior. Nowadays, this is one of the most difficult problems faced by nanotechnology. In fact, any advanced application that takes advantage of nanoparticle (NP) systems will also rely on the achievement of such control. This is true in particular for magnetic nano-particles that are unique and complex physical objects whose properties can greatly differ from their parent massive materials.