Photo absorption and photo dissociation studies of dimethyl sulphoxide and its deuterated isotopologue(DMSO-h₆ and DMSO-d₆) are performed using synchrotronradiation in the 35,000–80,000cm^-1 region. In the photo absorption spectrum, Rydberg series converging to the first three ionization potentials of DMSO at 9.1,10.1 and 12.3 eV corresponding to removal of an electron from the highest three occupied molecular orbitals (14a', 7a″ and 13a') are observed.Based on aquantumde fect analysis,Rydberg series assignments are extended to higher members as compared to earlier works and a few ambiguities in earlier assignments are clarified.Analysisis aided by quantum chemical calculations using the DFT and TDDFT methodologies.Vibronic structures observed in the spectrum of DMSO-h₆ in the regions 7.7–8.1eV and 8.1–8.8 eV are attributed to the transitions 7a″ →4p at 7.862 eV and 14a'→6s/4d at 8.182 eV, respectively. Photo absorption spectra of DMSO-h₆ and -d₆ recorded using a broadband incident radiation show prominent peaks, which are identified and assigned to electronic and vibronic transitions of the SO radical. This provides a direct confirmation of the fact that DMSO preferentially dissociates into CH₃ and SO upon UV–VUV excitation,as proposed in earlier photo dissociation studies.An extended vibronic band system associated with the e¹Π–X³Σ
transition ofthe SO radical is identified and assigned.The complete VUV photo absorption spectrum of DMSO-d₆ is also reported here for the first time.