Conformational stabilities of 1-methoxy-2-(methylthio)ethane were studied by matrix-isolation infrared spectroscopy, and the relevant intramolecular 1,5-CH...O interaction was examined by density functional calculations. The conformer with trans-trans-gauche(+/-) around the CH3O-CH2-CH2-SCH3 bonds is the most stable in an argon matrix and the conformer with trans-gauche(+/-)-gauche(+/-) is the second most stable. The energy difference between the two conformers was determined to be 0.7 +/- 0.4 kJ mol(-1) using thermal effusive sources. The 1,5-CH...O interaction is responsible for the high stability of the trans-gauche(+/-)-gauche(+/-) conformer, since the nonbonded (C)H...O distance associated with this interaction is shorter than the corresponding van der Waals separation by 0.15 Angstrom. The energy of the 1,5-CH...O interaction was evaluated to be 4.0 kJ mol(-1). (C) 2002 Elsevier Science B.V. All rights reserved.