Structure, lattice parameters, spontaneous magnetization ( $I_{s}$ ), and the Curie temperature ( $T_{\rm C}$ ) of MnAlGe and MnGaGe compounds with the Cu 2 Sb-type structure and their substituted compounds were investigated. Cr substitution for Mn enhanced $I_{s}$ and $T_{\rm C}$ , whereas Fe substitution for Mn degraded them. These behaviors are in accord with the previously reported results, and are also common to the MnGaGe compound series. For MnAlGe, lattice parameter $a$ increases by 0.2% and 0.4% for Cr and Fe substitution, while $c$ changes by +0.1% and −1.3%, respectively. For MnGaGe, $a$ decreases by 0.08% for Cr and increases by 0.2% for Fe substitution, while $c$ changes by +0.5% and −1.0%, respectively. $T_{\rm C}$ tended to increase with increasing length of $c$ , suggesting that the interlayer distance between Mn layers is a key factor related to the height of $T_{\rm C}$ , i.e., the strength of the magnetic exchange interaction.