Abstract The reactivities ( k 0 : the second-order rate constants of the reaction against singlet oxygen) of 11 reactive azo dyes derived from J-acid, L-acid, R-acid, 2R-acid and γ-acid on cellulose film immersed in an aerobic aqueous Rose Bengal solution and exposed to a carbon arc were estimated and analyzed in terms of frontier orbital theory using the semiempirical molecular orbital (MO) PM5 method. The azo–hydrazone tautomerism (AHT) was analyzed by calculating the standard enthalpies of formation for these dyes in the gas phase and water using the PM5 method. If a dye existed as the same tautomer in both the gas phase and water, it was regarded to exist as that tautomer on water-swollen cellulose; if it existed as different tautomers in the gas phase and water, it was regarded to exist as a mixture of tautomers on water-swollen cellulose. The k 0 values were confirmed to correlate to the electrophilic frontier density ( f r (E) ) of the tautomer on cellulose. The sum of f r (E) for the positions of reaction was confirmed to correlate linearly to log k 0 , although the correlation lines for the 11 dyes differed from the correlation lines reported previously for pyrazolinylazo and azobenzene dyes.