The gas-phase structure and conformational properties of bis(methylthio)methane, (MeS)(2)CH2, have been determined by electron diffraction, augmented by results from ab initio molecular orbital calculations. The molecule was found to exist in the gas phase at similar to 100 degrees C, predominantly in the G(+)G(+) form (70(18)%), where the CH3 groups an on opposite sides of the SCS plane, with a smaller contribution from the GA form in the conformational mixture. The main conformer possesses C-2 symmetry with a dihedral angle phi(CSCS) of 54(6)degrees, The second conformer has C-1 symmetry with CSCS dihedral angles of 74 degrees and 186 degrees (ab initio values). Assuming entropy differences between the two conformers as obtained from MP3/6-311+G(d) calculations (Delta S = R In 2 + 2.72 cal mol(-1) K-1), this composition corresponds to an enthalpy difference of Delta H = 2.2(6) kcal mol(-1). The corresponding ab initio/DFT enthalpy difference values at 373 K are 1.49 kcal mol(-1) (HF), 2.38 kcal mol(-1) (MP2), and 2.15 kcal mol(-1) (B3LYP). The results for the main distances (r(g)) and angles (angle(alpha)) from the combined GED/ab initio (HF/6-311+G(d)) study of the G(+)G(+) form of (MeS)(2)CH2 (with estimated 2 sigma uncertainties) are r(C-CH3-S) = 1.805(2) Angstrom, r(C-CH2-S) 1.806(2) Angstrom, r(C-CH3-H) = 1.108(5) Angstrom, r(C-CH2-H) = 1.098(5) Angstrom, angle(C-S-C)= 102.8(24)degrees, angle(S-C-S)= 115.9(3)degrees, angle(H-C-CH2-H) = 107.5 degrees (ab initio), and angle(S-C-CH3-H) = 108.9 degrees (ab initio).