The properties of the base acridine orange (AOB) in n-heptane/AOT/water reverse micelles were investigated by using absorption, fluorescence, and single photon counting techniques. For comparison, studies in homogeneous media (water and n-heptane) were also performed. The absorption spectra of AOB in water at pH < 10 in the range of [AOB] = 10(-6)-10(-4) M show two bands at 467 and 492 nm which were attributed to the dimer ((AOBH)(2)(2+)) and monomer (AOBH(+)) species, respectively. At pH > 10 in the same [AOB] range, only the basic form, AOB, was detected with a band in the visible at.,, = 435 nm, which obeys Lambert-Beer's law. This species was also the only one detected in n-heptane at 417 nm. These results show that only the protonated base dimerizes as confirmed by fluorescence techniques. The absorption spectra of AOB in the micellar media at various W-0 (W-0 = [H2O]/[AOT]) and pH = 4, working above the operational critical micellar concentration and below [AOT] = 3 x 10(-3) M, show the disappearance of the band originally present in n-heptane and the appearance of the bands attributable to the protonated base mostly as the dimer, (AOBH)(2)(2+). Above this concentration, the absorption band corresponding to AOBH(+) is the predominant one. There are two isosbestic points, at 426 and 475 nm. Three processes can account for the behavior of AOB in the reverse micelles: (1) distribution of the dye between the organic phase and the micellar interface followed by AOB protonation to give AOBH(+), (2) dimerization of AOBH(+) at the interface at low [AOT], and (3) the conversion of (AOBH)(2)(2+) to AOBH(+) by the micelle at [AOT] > 3 x 10(-3) M. The spectral changes allow us to estimate the equilibrium constants for the processes at different WO; the dimerization process is favorable at low water content. The fluorescence spectra of these species show at 650 nm the band for (AOBH)(2)(2+) and at 550 nm the one for AOBH(+). By varying the experimental conditions, the fluorescence decay times for AOB (in n-heptane and water) as well as for AOBH(+) and (AOBH)(2)(2+) in water and in AOT reverse micelles were determined. The results were used to explain the micellar influence on AOB's protonation and aggregation processes. The fluorescence decay times at low W-0 are higher than that obtained in bulk water. On increasing W-0, the fluorescence decay times tend to the value obtained in water at the same pH.