The thickness and internal properties of the magma sill located at the top of the axial magma chamber (AMC) along the southern East Pacific Rise (EPR) have been investigated through waveform modeling the near-vertical incidence reflections and analysis of reflection amplitude variation as a function of source-receiver offset. The results show that the AMC reflector observed along the southern EPR is best modeled by a thin (<100 m thick) sill of partial melt sandwiched between higher-velocity material, and that the thickest sills are generally associated with the lowest P and S wave velocities.