A new solvothermal method has been developed for the synthesis of monodisperse SmVO₄ and CeVO₄ nanocrystals with controlled size and shape. The obtained materials were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) techniques. The results reveal that uniform nanocrystals and pure tetragonal phase of SmVO₄ and CeVO₄ can be achieved. To investigate the oxidation states of the metals on the mixed oxide nanocrystal surface, the XPS technique was employed. The results exhibit that only one oxidation state of samarium, cerium, and vanadium for each metal (e.g., Sm³⁺, Ce³⁺, V⁵⁺) was surprisingly well stable on the particle surface at the nanoscale, even after calcination, while the existence of two oxidation states of these metals is observed (e.g., Sm³⁺/Sm²⁺, Ce⁴⁺/Ce³⁺, V⁵⁺/V⁴⁺) in the corresponding single metal oxide nanocrystals.