A joint inversion of both P-wave travel-time and polarization direction data is performed on a data set from the South Iceland Lowland network. The root mean square reduction of the polarization and travel-time residuals are 72% and 76%, respectively. Furthermore, travel-time residuals drop 45% even when they were given no weight in the inversion, indicating that the polarization measurements are related to the same regional-scale velocity heterogeneities thought to cause the travel-time anomalies. The results show two crustal low-velocity zones trending NE across the study area, a direction that is sub-parallel to the strike of the Western Volcanic Zone in south Iceland. ¿ American Geophysical Union 1993