Excited state proton transfer (ESPT) of an anionic photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS or pyranine) has been studied inside a cationic reverse micelle (RM), water/benzylhexadecyldimethylammonium chloride (BHDC)/benzene, using steady-state and time-resolved fluorescence spectroscopy. The observed ESPT behavior is found to be remarkably different from the known ESPT trend of HPTS inside anionic AOT and cationic CTAB RMs; the ESPT dynamics approaches that of bulk water at higher w(0) (>= 10) inside AOT RM while no ESPT was observed for CTAB reverse micelle [Sedgwick et al. J. Am. Chem. Soc. 2012, 134, 11904-11907]. The ESPT dynamics inside BHDC RM is remarkably slower compared to that of water at all w(0) (= [water]/[surfactant]) values and relatively much less sensitive to w(0) variation compared to AOT RM. 2D NOESY and fluorescence anisotropy measurements reveal that the probe (HPTS) is embedded inside the positive interface of BHDC P.M. Despite its trapped location, HPTS is able to undergo ESPT due to significant penetration of water molecules into the interface. Furthermore, facile ESPT at higher w(0) is consistent with higher degree of interface hydration as predicted by a recent MD simulation [Agazzi et al. Langmuir 2014, 30, 9643-9653]. The study shows that ESPT dynamics inside P.M varies not only with the interface charge but also on the nature of the headgroup and solvation.