Isentropic compression experiments were performed on molten tin (initial temperature 500–600 K), using the Sandia Z Accelerator to generate magnetically driven, planar ramp waves compressing the tin across the equilibrium liquid‐solid phase boundary. Velocity interferometry measured time‐resolved wave profiles at the tin/window interface. The experiments exhibit a departure from expected liquid response, time‐dependent behavior above 8 GPa, and, at higher pressure, reduced wave speed relative to calculations using a nonequilibrium phase‐mixture model. These phenomena may be due to a nonequilibrium solidification process, but verification of this conjecture will require further work. © 2004 American Institute of Physics