Abstract: The fluoroscent dyes are used as probes for the detection of heavy metal ions, aminoacids and other biomolecules in recent years. Some of these dyes, including Nile Blue deivatives, which belong to the benzophenoxazine family, fluoresce at near the far visible-near infrared region with minimum interference from absrption scattering and reduced risk of photodecomposition.¹ As previously reported by our group,^2,3 benzo[a]phenoxazinium dyes are involved in an acid-base equilibra where the 5-amino position is the main site of deprotection.⁴ The quantum yield of the basic form is almost ten times lower than the cationic acid form. This has an impact in the overall fluorosence quantum yield. In continuation of our research intrests,2-4 the present work describes the study of fluoresence quantum yileds of pure cationic forms by shifting the acid-base equilibria with the addition of a strong acid. The photophysical behaviour of these novel compounds in AOT/cyclohenxane/water w/o microemulsions as a function of water content will also be discussed. Acknowledgements: Thanks are due to Fundacao para a Ciencia e Tecnologia (Portugal) for its financial support of a BPD to B. R. Raju (SFRH/BPD/62881/2009), Centro de Quimica and Centro de Fisica (Universidade do Minho). The NMR Spectro meter Bruker Avance III 400 is part of the National NMR Network, and was purchased in the framework of the National Program for Scientific Re-equipment, contract REDE/1517/RMN/2005 with funds from POCI 2010 (FEDER) and FCT. Refrences: 1. Goncalves, M. S. T. Chem. Rev. 2009, 109, 190. 2. Alves, C. M. A.; Naik, S.; Coutinho, P. J. G. Goncalves, M. S. T. Tetrahedron 2009, 65, 10441. 3. Alves, C. M. A.; Naik, S.; Coutinho, P. J. G. Goncalves, M. S. T. Tetrahedron Lett. 2011, 52, 112. 4. Naik, S.; Alves, C. M. A.; Coutinho, P. J. G. Goncalves, M. S. T. Euro. J. Org. Chem. 2011, 13, 2491.