The molecular structure and electronic spectra of the orange and blue isomers of the photochromic compound dithizonatophenylmercury(H) were theoretically studied utilizing density functional (DFT) methods. Computed structural results are in agreement with previously reported X-ray crystal data of the orange resting state. The herewith newly proposed geometrical structure of the blue photoexcited state is favored by more than 35 kJ center dot mol(-1) relative to the historically hypothesized geometry of the blue isomeric form. The key difference lies in the position of the backbone amine proton, being situated on the N4 position in the newly proposed structure, rather than on the N2 position as in the previously hypothesized geometry. Time dependent density functional theory as implemented in the Amsterdam Density Functional (ADF) and Gaussian 03 (G03) program systems yielded excitation energies for the blue isomer exhibiting bathochromic shifts, as observed in the experimentally determined UV/visible spectrum. B3LYP calculated excitation energies and oscillator strengths gave the best approximation of the experimentally observed electronic spectra of both isomers.