The SHARAD instrument on the Mars Reconnaissance Orbiter detects subsurface interfaces beneath lava flow fields northwest of Ascraeus Mons. The interfaces occur in two locations; a northern flow that originates south of Alba Patera, and a southern flow that originates at the rift zone between Ascraeus and Pavonis Montes. The northern flow has permittivity values, estimated from the time delay of echoes from the basal interface, between 6.2 and 17.3, with an average of 12.2. The southern flow has permittivity values of 7.0 to 14.0, with an average of 9.8. The average permittivity values for the northern and southern flows imply densities of 3.7 and 3.4 g cm(-3), respectively. Loss tangent values for both flows range from 0.01 to 0.03. The measured bulk permittivity and loss tangent values are consistent with those of terrestrial and lunar basalts, and represent the first measurement of these properties for dense rock on Mars. Citation: Carter, L. M., B. A. Campbell, J.W. Holt, R.J. Phillips, N.E. Putzig, S. Mattei, R. Seu, C. H. Okubo, and A. F. Egan (2009), Dielectric properties of lava flows west of Ascraeus Mons, Mars, Geophys. Res. Lett., 36, L23204, doi: 10.1029/2009GL041234.