A good understanding of the relationship between the atomic scale structure of ultrasmall europium oxide nanocrystals (NCs) and their photoluminescence properties is of major interest in the design and development of innovative europium-based nanophosphors. As a consequence, the preparation of reliable (controlled size and shape distributions) and structurally well characterized ultrasmall europium oxide NCs is an essential prerequisite to understand the size effects on their photoluminescence properties. First, we reveal that nonaqueous approaches used to synthesize ultrasmall europium oxide NCs are deeply affected by "hidden" parameters that are directly related to the preparation of the reactive mixture. Indeed, trace amounts of products of side reactions and byproducts, such as acetic acid and water, act as growth-directing agents. Second, the challenging problem related to the structural characterization of ultrasmall europium oxide NCs is addressed for the first time by coupling high-resolution transmission electron microscopy and X-ray atomic pair distribution function. The ultrasmall thickness of the as-prepared NCs apparently dictates the crystalline structure, which can no longer be described by the crystal phases of their bulk counterparts. The induced distortions due to the ultrasmall thickness as well as the bonding of the stabilizing organic ligand are strong enough to break down the symmetry and, hence, prevent the europium oxide NCs from accommodating the usual bulk crystal phase. Finally, the formation of such unusual polymorphs of europium oxide has a profound impact on the resulting crystal field, with direct effects on the photoluminescence properties.