The structure and magnetic properties of magnetite (Fe3O4) nanoparticles synthesized by a solvothermal processing route are investigated. The nanoparticles are grown from the single organometallic precursor Fe(III) acetylacetonate in trioctylamine (TOA) solvent at 260 °C, with and without the addition of heptanoic acid (HA) as a stabilizing agent. From the temporal particle size distributions, x-ray-diffraction patterns, high-resolution transmission electron microscope tilt series experiments, and superconducting quantum interference device magnetometry, we demonstrate that HA, a strong Lewis acid stabilizing agent, slows growth processes during ripening thus reducing the formation of interfacial defects, which we observe in the TOA-only synthesis. Nanoparticles grown with HA remain single crystalline for long growth times (up to 24 h), show a focused particle size distribution for intermediate growth times (3 h), and possess a higher magnetic anisotropy (15.8×104 J/m3) than particles grown without the additional stabilizing agent. The reduced magnetic anisotropy value for the magnetite nanoparticles grown in TOA only (1.29×104 J/m3) is attributed to polycrystallinity induced by the uncontrolled ripening process. This work may have significance for contrast enhancement in magnetic resonance imaging.