The materials used as media for volumetric storage by means of two photon absorption processes have been studied. The spectroscopic properties, transient spectra and kinetics of several composite molecules are described. The media consist of a photochromic moiety that is chemically bonded to a strongly fluorescing dye. Experimental data presented here show that this composite molecule performs as a write/read/erase molecular optical storage medium with non-destructive read-out. The optical head used to record and access the stored information is also depicted.