Mesoporous thin films comprised of ∼15 nm diameter SnO₂ nanocrystallites were synthesized and characterized in acetonitrile electrolytes by electrochemical and spectroscopic techniques. Spectroelectrochemical reduction of the thin films resulted in broad, non-superimposable UV/vis absorption changes. Simultaneous analysis of potential-dependent spectra, by a process termed “potential associated spectra”, resulted in the identification of three unique absorption spectra for reduced SnO₂, while only one spectrum was identified for TiO₂. Reduction of SnO₂ resulted in the appearance of (1) a broad absorption that spans across the visible and near-IR regions, (2) a blue-shifted fundamental absorption, and (3) an absorption band in the blue region. The absorption onsets were dependent on the electrolyte cation, present as the perchlorate salt of Li⁺, Na⁺, Mg²⁺, Ca²⁺, and TBA⁺, where TBA⁺ is tetrabutylammonium. Correlations between the charge within the thin film and the absorbance intensity revealed that significant charge was transferred to SnO₂ films before significant visible color changes were observed. This suggested the presence of electrons within the SnO₂ thin films that did not absorb visible light and were termed “phantom electrons”.