Eu(III) and Tb(IV) activators were embedded in quantum size SnO₂ nanoparticles by a hydrothermal method and their morphologies, crystal structures and spectroscopic properties were examined by transmission electron microscopy, X-ray diffraction crystallography, UV-visible absorption, and 2D/3D-photoluminescence mapping. Eu(III) embedment showed strong red emissions between 570 and 720 nm, which were attributed to the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transitions. The magnetic dipole ⁵D₀ → ⁷F₁ transition was dominated by an indirect charge transfer excitation, which was attributed to Eu(III) doped at centrosymmetric sites. Conversely, the electric dipole ⁵D₀ → ⁷F₂ transition was dominated by direct excitation of Eu(III), which was attributed to Eu(III) at low symmetry sites. Tb(III) was oxidized to Tb(IV) in SnO₂ and showed no characteristic green emissions. A full understanding of the photoluminescence mechanism of Eu(III) and Tb(IV) in SnO₂ will provide new insights into the development of SnO₂-based phosphor materials.