The conformations and molecular structures of diisopropyl disulfide have been studied by high-resolution microwave spectroscopy and quantum chemical calculations. Three conformers, G'GG', G'GT, and GGG', have been observed in the jet expansion. The global minimum, G'GG', adopts a configuration with the G' orientation of H-C-S-S and S-S-C-H and the G orientation of C-S-S-C showing the C-2 symmetry. The rotational spectra of monosubstituted C-13 and S-34 isotopologues have also been recorded for G'GG', leading to an accurate structural determination of this conformer. Two additional 34S isotopologues have also been measured for G'GT. The relative energies of three observed conformers calculated at the MP2/6-311++(d,p) level of theory are within 2 kJ mol(-1), while the relative intensity measurements suggested their population ratio to be N-G'GG'/N-G'GT/N-GGG' approximate to 5:3:2.