It has long been argued but never demonstrated that alteration "fronts" should be recognizable features of the seismic structure of the oceanic crust. The abrupt transition from crust affected by low-temperature hydrous alteration to crust affected by high-temperature hydrothermal alteration must arise from a stepwise reduction of porosity and permeability that should correspond to a seismic boundary. In Holes 504B and 1256D, the sudden downhole appearance of hydrothermal minerals corresponds to a increase of the velocity gradient that is caused by a change of porosity within the lava-dike transition zone, and models of the downhole variation of permeability computed from apparent porosity logs show a corresponding stepwise change of permeability (by a factor of approximately 20) that is sufficient to account for the onset of hydrothermal alteration. In principle, the coincidence of the seismic structure with the alteration boundary can be used to interpret the seismic structure of the oceanic crust. In particular, the onset of hydrothermal alteration proves to be a viable candidate for the transition from Layer 2A to Layer 2B, which also occurs within or near the lava-dike transition zone. There is also a systematic decrease of permeability with increasing sonic velocity in both the lavas and the dikes (log(kappa ) approximately a + bv). Remarkably, the extrapolated trend for the lavas is in excellent agreement with in situ permeabilities measured in very young crust, ranging in age from zero to 3.5 Ma. To a good approximation, the permeability of Layer 2A can be estimated from its seismic structure.