Abstract The harmonic force field for S 2O has been determined from a least-squares refinement to an extensive data set, which included vibrational wavenumbers, isotopic shifts and centrifugal distortion constants. Principal force constants are k R (SS) = 4.610 (39) aJ Å −2, k r(SO) = 8.583(70) aJ Å −2 and k α(SSO) = 1.326(6) aJ rad −2. The value of ν 3 measured in an inert gas matrix appears incompatible with the remaining gas-phase data. Standard ab initio DZ (P)/SCF calculations predict the molecular structure of S 2O quite satisfactorily, but their predictions concerning the force field are of uneven quality, especially for the off-diagonal elements. The r z molecular structure S 2O has been obtained, and is compared with the r 0, r s and r e representations. Bond lengths and force constants for related sulphur-containing molecules are compared, and correlations with the apparent participation of d orbitals on sulphur are discussed.