The compressional wave velocity in molten iron containing 5 percent nickel and 10 percent sulfur increased with increasing temperature. This anomalous behavior was determined to be attributable to-the presence of sulfur, which conditions the formation of large molecular units in the liquid. Temperature-induced breakup of macromolecular units resulted in greater packing efficiency and an increased velocity. Ten percent sulfur increased attenuation by one to two orders of magnitude compared with liquid iron, Such behavior at outer core pressures and temperatures would constrain the velocity gradient in the outer core and would enable the discrimination of potential light alloying elements.