When mixed with water, n-octyl beta-n-glucoside forms self-assembled nanostructures, several of which are liquid crystalline and all of which depend on the water/glucoside ratio and temperature. For practical use of these phases, a detailed understanding of the conditions under which they exist (i.e., the isobaric phase diagram) is required. We use the fluorescence of the dye molecule prodan as a new approach to probe the phases formed in these mixtures. The prodan fluorescence signal depends on the polarity of its environment and thus the phase(s) in which the dye exists. Visual inspection of the total fluorescence signal can qualitatively determine the phases present, including coexisting phases. Temperature-induced phase changes are also detected from variations observed in the prodan fluorescence spectrum. The sensitivity of this new technique allows the single- and multiple-phase regions to be mapped carefully for the first time.