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Spectral characteristics of 2-(4'-N,N-dimethylaminophenyl)pyrido[3,4-d]imidazole (DMAPPI) have been studied in two different solvents and H(0)/pH/H(-) range of -10 to +16. Combining the results observed in the absorption, fluorescence, and fluorescence excitation spectra, it is found that (i) only one kind of monoanion (deprotonation of >N-H group) and one kind of trication, (ii) two kinds of monocations (MC1 and MC3) in the ground state and (MC2 and MC3) in the excited singlet state, and (iii) three kinds of dications (DC1, DC2, and DC3) are observed. Semiempirical quantum mechanical calculations (AM1) have been carried out on all kinds of ionic species and their resonating structures. The spectral characteristics have been assigned to various prototropic species combining the experimental and theoretical results.