Recently, 4.4'-bis(1-p-carboxyphenyl-3-methyl-5-dydroxyl)-pyrazol (DRD156) has been developed as a new sensitive reagent that reacts specifically with singlet oxygen. The specificity of DRD156 for singlet oxygen in a biomimetic solution (micellar solution) and the effects of its coexistence with other reagent were examined with electron spin resonance (ESR). Singlet oxygen was generated using photosensitization reaction. The ESR spectrum of the radical derived from DRD156 after the reaction with singlet oxygen in phosphate buffered salines (PBS) was comprised of twenty-nine lines, whereas that in cetyltrimethylammonium bromide (CTAB) micelles was comprised of nine lines. Both 2,2,6,6-tetramethyl-4-piperidine (TMPD) and 1,3-diphenyl-isobenzofuran (DPBF) reduced the singlet oxygen-DRD156 signal intensity, and TMPD-mediated decrease in PBS (to 62%) was almost the same as that in CTAB micelle (to 65%). In contrast, DPBF reduced the DRD156 signal intensity more effectively in CTAB micelle (to 12%) than PBS (to 38%). These results indicate that the specificity of DRD156 for singlet oxygen is dependent on microenvironment.