First-order calculations of spin-orbit constants, dipole moments and carbon-sulphur distances have been performed for HCnS (n = 1-12) radicals in the 2 electronic ground state. It is found that these molecular properties alternate with even or odd numbers of carbon atoms in the chains and the spin-orbit constant ASO is around -300 cm-1 for n even and about +120 cm-1 for n odd throughout the series. This agrees with the experimentally determined value of about -270 cm-1 for HC2S, but the theoretically predicted ASO values are much larger than the values given for HC3S and HC4S from a fit of their millimeter-wave spectra. Values that were too low were also assumed in the analysis of the rotational spectra of n = 4-8.