The dynamics of exchange between amphiphile assembled in reverse (water-in-oil) micelles and unimer (free in solution) amphiphile were probed by C-13-NMR spectroscopy in a ternary isothermal system consisting of a poly(ethylene oxide)-poly(propylene oxide) block copolymer (poloxamer), water, and xylene, An upper bound was obtained for the lifetime of a block copolymer molecule within the reverse micelle, using the theoretical treatment of exchange phenomena in NMR which is based on extensions of the Bloch equations.