The structure and spectroscopic properties of Phenol Blue [N-(4-dimethylaminophenyl)- 1,4-benzoquinoneimine (4)], which can exist in principle either as a zwitterion (A) or as a quinoneimine (B), have been assessed both experimentally and theoretically using molecular orbital methods. C-13 NMR evidence on the more soluble diethylamino derivative (6) strongly suggests that the molecule exists purely as the quinoneimine (B) both in protic solvents and in aprotic solvents of low and high dielectric constant. Theoretically, the solvatochromic shift of the dye in aprotic solvents, calculated using the PM3/COSMO method, shows a good correlation with the experimental data and arises because solvents with large dielectric constants exert a much greater stabilizing influence on the more polar excited state than they do on the ground state. In protic solvents, the larger bathochromic shifts observed are attributable to both a dielectric effect and a separate hydrogen bonding contribution from the solvent. In water, Phenol Blue is predicted to form a stable trihydrate which is calculated at the CNDOVS level of theory to absorb at a significantly longer wavelength than the unsolvated structure in line with the experimental data.