The crystal structure and thermoelectric properties of type-III clathrate compounds in the Ba-In-Ge system have been investigated as a function of In content. The solid solubility of In in the type-III clathrate compounds is determined to be about X=15 when expressed with the formula of Ba24 InX Ge100-X. Similar to our recent results obtained for type-III clathrate compounds in Ba-Al-Ge and Ba-Ga-Ge systems, values of electrical resistivity, Seebeck coefficient, and electronic thermal conductivity of those in the Ba-In-Ge system generally increase with the increase in the In content because of the decrease in the number of excess electrons as described with the simple Zintl concept. The changes in their lattice thermal conductivity with the In content are explained in terms not only of the low rattling frequency of the guest atom but also of the low Debye temperature of the cage framework. © 2007 American Institute of Physics.