The effect of Mo substitution at Ti site of orthorhombic Sm5 Ge4 -type R2 Ti3 Ge4 compounds on the magnetic and electrical transport properties has been studied. The Tb2 Ti3-x Mox Ge4 (x=0.3,0.75) and Er2 Ti2.7 Mo0.3 Ge4 compounds have been synthesized and it is found that these compounds retain parent crystal structure at room temperature (space group Pnma, No. 62). Mo substitution decreases the antiferromagnetic ordering temperature (TN) of Tb2 Ti3 Ge4 compound from ∼18 to ∼13 and ∼10 K, respectively, for x=0.3 and 0.75. The Er2 Ti2.7 Mo0.3 Ge4 compound shows a tendency to order at ∼2 K, whereas the parent Er2 Ti3 Ge4 is magnetically ordered at 3 K. Magnetization versus field data of Tb2 Ti3-x Mox Ge4 (x=0.3,0.75) reveal soft ferromagnetic nature. The metamagnetic transition that is present in parent Tb2 Ti3 Ge4 is found to disappear with Mo substitution. Magnetization value reaches ∼6.2 μB / Tb3+ at 2 K in fields of 8 T, indicating incomplete ferromagnetic ordering with or without an antiferromagnetic component. Electrical resistivity of the Tb-based compounds has a linear variation with temperature from 300 to ∼50 K and shows a prominent slope change at temperatures much above TN, supporting the presence of competing short range ferromagnetic interactions. © 2009 American Institute of Physics.