Viscoelastic properties of basalt melts of a wide range of composition are determined from ultrasonic compressional wave velocity Vp and quality factor Qp measurements in the frequency range of 3.4-22 MHz and the temperature range of 1150¿--1500¿ C. The basalt samples are grouped into two types: low magnesia (group A) containing 5--9 wt % MgO and high magnesia (group B) containing 18--23 wt % MgO. The Vp values of the melts determined at 1400¿ C are in the range 2.53--2.64 km/s for group A basalts and 2.79--2.86 km/s for group B basalts. The relaxed (low frequency) velocity Vr is a principally linear function of the melt density &rgr;. Least squares fits of the data give velocity-density relations Vr (km/s)=0.691+0,712&rgr; (g/cm3) for group A basalts and Vr=0.328+0.907&rgr; for group B basalts at 1400¿ C. The mean molecular weights M are 65.0¿0.3 and 58.2¿0.5 g/mol for group A and B basalts, respectively, suggesting that M is also a principal factor determining the melt velocity. The relaxed modulus Mr of the basalt melts range from 16.5 to 22.8 GPa and is also a linear function of the melt density. The values of longitudinal &eegr;L and volume &eegr;v viscosities are calculated from the measured Vp and Qp and the available shear viscosity &eegr;s data. The viscosities and related activation energies decrease almost linearly as NBO/T (nonbridging oxygens per tetrahedrally coordinated cation) increases.