We aim to find out the extent of stability of the excimer of 9-aminoacridine hydrochloride hydrate (9AA), a prospective PDT drug, in different confined media with varying cavity size. When confined in cetyltrimethyl ammonium bromide micelles, although at low concentration of 9AA, only a single distinct peak (lambda(max) at 460 nm) with a shoulder at 485 nm is observed in steady-state fluorescence spectrum, yet with increase in concentration the peak and the shoulder merge with simultaneous emergence of another peak at 535 nm, which is assigned to excimer. Similar behavior is also observed in Triton-X, crown ether, a-cyclodextrin, beta-cyclodextrin, and homogeneous aqueous medium. The formation of excimer, which reflects the extent of confinement of 9AA, is maximum in beta-cyclodextrin followed by others. Steady-state and time-resolved fluorescence studies along with TRES and TRANES analyses coupled with anisotropy data and transient absorption studies reveal the presence of monomer dimer equilibrium of 9AA in the excited state. Molecular modeling indicates that the structure of excirner is stabilized by locking of the two monomeric species via four hydrogen bonds formed between the amino-H and imino-N of 9AA monomers, whereas the dimer in the ground state has only two such hydrogen bonds.