The crystal structures of some type-I and -III clathrate compounds in the Ba-Ga-Ge system have been investigated by synchrotron X-ray powder diffraction at room temperature, paying special attention to the changes of the cage structure and the splitting behavior at the guest atom site upon alloying with Ga. For both types of the clathrate compounds, the split distance of the Ba(2) sites increases with the increase in the Ga content, corresponding to the change in the size and shape of the encapsulating polyhedral cage. Lattice thermal conductivity at room temperature has a positive correlation with the atomic displacement parameter (ADPsplit) based on the split-site model but has an inverse correlation with the split distance of the Ba(2) sites, indicating that a dominant factor of reducing the lattice thermal conductivity is not thermal vibration at the split sites but thermal jump among the split sites. © 2006 Materials Research Society.