A transition phase can be defined as a structure formed at relative low temperature, irreversibly transforming at higher temperature into an equilibrium phase of the same elemental composition Among the transition aluminas, gamma-alumina, usually produced from boehmite, is an extremely important technological material. It is easy to produce gamma-alumina by sol-gel technique. This alumina, which is formed by firing the xerogel at temperature range between 500 to 800 °C, has a nanocrystalline cubic spinel-like structure entirely different from the rhombohedral hR10 structure of alpha-alumina, that formed by thermal treatment at higher temperatures. This work deals with nanocrystalline M2O3 oxides formed by firing xerogels at relatively low temperatures. The following nanocrystalline binary oxide systems were studied: MgO-TiO2, ZnO-TiO2, NiO-TiO2, ZrO2-Al2O3, and Y2O3-Ln2O3, and Y2O3-Ln2O3 (Ln = rare earth element). Some pure or doped unary oxides were also studied. The xerogels were heated at constant T (200 to 1600 °C) for 3 to 6 hours. There was a threshold temperature for oxide formation and in many cases the products were transition nanocrystalline phases, depending on grain size and composition, including doping. The binary MM’O3 oxide phases of NiTiO3, MgTiO3, and ZrTiO3 are ilmenite type structures with rhombohedral hR10 structure similar to alpha-alumina, however since they have more than one component it is possible to study nanocrystalline solid solutions. It was found that firing the xerogels at temperature range between 300 and 600 °C transition nanocrystalline phases were obtained. The phases have several cubic types. In contrast to the ilmenite-like compounds (NiTiO3 for example) which are stoichiometric, the transition phases exist at wide range of non-stoichiometric compositions. A model correlating the size effect with the unusual solid solutions and structures is proposed, correlating the size effect with the structure and enhanced solubility.