Phenothiazine dyes are known to inactivate bacteria in whole blood and plasma caused by the singlet oxygen photo generated by these dyes. Methylene blue (MB), 1,9-dimethyl-methylene blue (DMB) and toluidine blue (TB) transient singlet and triplet states spectra and their formation and decay kinetics have been measured by time resolved spectroscopy. The triplet state formation and singlet oxygen quantum yields in water are found to be approximately the same in MB and DMB. Therefore, based on our data we propose that although singlet oxygen is highly important as previously stated, the rate of inactivation is determined by the binding of the dye to the bacteria. (C) 2010 Elsevier B.V. All rights reserved.