Diethyl disulfide was investigated by pulsed jet Fourier transform microwave spectroscopy. The spectroscopic study was complemented by ab initio calculations. The first two most stable conformers predicted at the MP2/6-311++G(d,p) level of theory were observed in the supersonic expansion. Two ' C-13 and one S-34 isotopologues for the most stable conformer (GGG) and two S-34 isotopes for the second most stable form (GGG') were also assigned in natural abundance, respectively, allowing a thorough investigation of the molecular structure and conformation. The relative population ratio of the two conformers in the supersonic expansion was estimated to be about 4:1.