The tautomerism of dihydroxynaphthoquinone dyes 1 and 2 were studied by means of molecular orbital calculations and 1 H NMR analyses. Molecular orbital calculations showed that the most stable tautomers of dyes 1 and 2 are 1,5-quinone and 1,4-quinone structures, respectively. These results are in good agreement with those of 1 H NMR and X-ray analytical data. The ab initio calculations carried out with the density functional method were advantageous for determining the structure of the most stable tautomer. Dye 1 showed a bathochromic shift of 100 nm in absorption maximum from the solution to solid state. Molecular orbital calculations were used to explain this bathochromic shift.