Hydrothermal reactions of molybdenum(III) oxide with isoquinoline under different pH values lead to two new octamolybdate-based polyoxometalates (POMs), namely, (C 9 H 7 NH) 4 (C 9 H 7 N) 2 [Mo 8 O 26 ] (I) and (C 9 H 7 NH) 4 [(Mo 8 O 26 )(C 9 H 7 N) 2 ]·2(H 2 O) (II), and their structures were determined by single-crystal X-ray diffraction analysis. The two compounds represent the first report of [Mo 8 O 26 ] 4-based POMs associated with isoquinoline ligand. Compound (I) has 3D supramolecular framework based on hydrogen bonds between [β-Mo 8 O 26 ] 4-fragment and protonated isoquinoline (C 9 H 7 NH) + . Compound (II) contains [(γ-Mo 8 O 26 )(C 9 H 7 N) 2 ] 4-units constructed by [γ-Mo 8 O 26 ] 4-units and isoquinoline through Mo-N bonds.