The Seebeck coefficient, electrical resistivity, and thermal conductivity of Zr3Mn4Si6 and TiMnSi2 were studied. The crystal lattices of these compoundscontain relatively large open spaces, and therefore, they have fairly low thermal conductivities (8.26 and 6.63 Wm -1 K -1 , respectively) at room temperature. Their dimensionless figures of merit ZT were found to be 1.92 × 10 -3 (at 1200 K) and 2.76 × 10 -3 (at 900 K), respectively. The good electrical conductivities and low Seebeck coefficients may possibly be due to the fact that the distance between silicon atoms in these compounds is shorter than that in pure semiconductive silicon.