Langmuir-Blodgett (LB) films of the hemicyanine dyes 4-(4-dihexadecylaminostyryl)- N-methylpyridinium iodide and 4-(4-dioctylaminostyryl)-N-propylsulfonate pyridinium mixed with octadecanoic acid have been deposited onto H2O-swollen poly(2-vinylpyridine) (P2VP) as mass transport barriers. The transport properties of these structures have been studied both with and without the application of an electric field in the range 0 ≤ E ≤ 100 V/cm, by observing the rate of permeation of both anionic (fluorescein) and zwitterionic (rhodamine B) fluorophores. The apparent diffusion coefficient at E = 0 decreases exponentially with the number of monolayers through which the probe molecule must permeate, in agreement with a hole-mediated permeation model. Application of an external electric field is observed to control the rate and direction of the mass transport of the probe molecule across the barrier layer. The electric field effect is both spatially reversible, i.e. the probe can be transported in either direction across the barrier layer, and temporally reversible, i.e. the permeability cycles with the field between large values when the field is on and small values when it is not applied. The results are consistent with an interpretation in which electroporation is the dominant mechanism for transport in the field-on state.