The ground-state rotational spectra of two isotopomers, (CH 2 ) 2 3 2 S HC 1 4 N and (CH 2 ) 2 3 2 S HC 1 5 N, of a complex formed by thiirane with hydrogen cyanide were analysed to give rotational constants, centrifugal distortion constants and 1 4 N-nuclear quadrupole coupling constants. The spectroscopic constants were interpreted in terms of a geometry of the complex characterised by the angle = 89.2(9)° made by the S H internuclear axis with the thiirane heavy atom plane at S and a nonlinearity θ 43° of the S H-C hydrogen bond. This large nonlinearity is contrasted with that in (CH 2 ) 2 O HCN and the difference discussed in terms of a simple model of the hydrogen bond.