We have studied the structural, electronic and vibrational properties of the clathrates Ba8Al16Ge30 and Ba 8Al16Si30 using the local density approximation (LDA). The equilibrium structures that we have obtained for these materials show that the Si-containing compound Ba8Al16Si 30 is energetically more stable than its Ge counterpart Ba 8Al16Ge30 by -0.38eV per atom. We also find that Ba8Al16Si30 is a narrow gap semiconductor with an indirect LDA energy band gap of 0.37eV along the Γ-M high symmetry direction in the Brillouin zone and that Ba8Al16Ge 30 is a direct band gap semiconductor with an energy gap of 0.43eV. We have also calculated the lattice vibrational modes of these materials and have identified the low frequency 'rattler' modes produced by the motion of the Ba atoms that are loosely bound in the cages of the Al-Ge or Si framework. These low lying Ba modes are thought to play a part in reducing the thermal conductivities of these materials.