Three new indium(III) compounds, ln(HCOO)(3) (1), ln(2)(HCOO)(5)(OH) (2), and ln(HCOO)(2)(OH) (3), were synthesized under hydrothermal conditions and characterized by single crystal and powder X-ray diffraction experiments, as well as by IR spectroscopy, elemental analysis, and coupled TG-DSC-MS measurement. All of these compounds adopt 3D framework structures consisting of InO6 octahedra and the 2.11 binding modes of formate with the (syn, syn-; syn, anti-; anti, anti-) configurations. The structural investigation of these indium formates reveals that the gradual introduction of the hydroxyl groups into the structures induces the polymerization of the InO6 octahedra, that is, InO6 is isolated in 1, becomes dimeric in 2, and finally forms 1 D chains in 3. In addition, a simple formula that may be used for estimating the overall coordination number of the formate in M-a(HCOO)(b)L-c is proposed.