Ultrasonic measurements of compressional wave velocities for six komatiitic melts containing 13--30 wt% MgO are reported in the frequency range 3--20 MHz and in the temperature range 1280--1661 ¿C. Melts with less than about 20 wt% MgO have velocities that are 6% lower and relaxed compressibilities, &bgr;r, that are 15% higher than melts with more than about 20 wt% MgO. The general trend of increasing velocity with increasing MgO content correlates well with increasing depolymerization (decreasing molar volume) of the melts as reflected in the NBO/T, (CaO+MgO)/Al2O3, and Al/(Al+Si) ratios. The value of d&bgr;r/dw (w=wt% MgO) for the high MgO melts is more than four times greater than &Bgr;r/dw for the low MgO melts. ¿ American Geophysical Union 1991