Rotational and centrifugal distortion constants were determined for eight isotopomers of a weakly bound complex of thiirane and ethyne. The observed complex is of C-s symmetry, with ethyne lying in the symmetry (ab principal inertial) plane and with H of HCCH forming a hydrogen bond to the S atom of thiirane. The r(0) geometry is: r(S...H)=2.76(5) phi, phi=84.0(5)degrees, and theta=42.9(2)degrees, where phi is the angle made by the S...H internuclear line with the local C-2 axis of the thiirane ring and theta is the angular deviation of the S...H-C nuclei from a linear arrangement. The nonlinearity of the hydrogen bond is attributed to a weak primary interaction S...H-C coupled with a secondary interaction involving the pi bond of ethyne with the two equivalent H atoms of thiirane that are closest to it. The angular geometry of (CH2)(2)S...HCCH is compared with those of other species (CH2)(2)S...HX, where X=F, Cl, Br, and CN. (C) 2003 American Institute of Physics.