Internal rotation in 1,4-disubstituted but-2-ynes XCH2CCCH2Y (X, Y = H, CH3, Li, F) has been studied with the aid of ab initio molecular orbital theory. When either or both of X and Y are approximately electroneutral (H or CH3), the barrier to internal rotation is very small. On the other hand, when X and Y are both electropositive (LiCH2CCCH2Li) or both electronegative (FCH2CCCH2F), there are pronounced preferences for conformations in which the dihedral angle XC·CY is slightly greater than 90°. When X is electropositive and Y electronegative (LiCH2CCCH2F), the stable rotational isomers are cis and trans with the former being preferred. For all molecules examined here the threefold component of the torsional potential function is very small.