The structural transformation of ZnS nanowires in high vacuum conditions induced by either atomic O beam treatment or electron beam it-radiation has been studied. Interestingly, we observed that both types of treatments produce a higher rate of "hollowing" in the core of the wire than in its peripheral walls. Depending on the substrate temperature, atomic O beam treatment can either result in (i) the conversion of ZnS into ZnO nanotubules with similar dimensions due to incomplete structural relaxation or (ii) conversion into photoluminescent, nanocrystalline ZnO wires with dimensions much smaller than the starting ZnS nanowires. The observed morphological changes at different temperatures were due to the relative rate of evaporation of ZnS and its oxidation into thermally stable ZnO in the vacuum. Periodic density functional theory calculations were performed to derive insights into the contraction of the surface bonds and the unit cell volume following the substitution of S by O. Our calculations show that increasing the O content in the ZnSxO1-x alloy results in a monotonic decrease in the unit cell volume, while surface oxygenation leads to compressive stress, which is correlated to the elastic instability of the oxygenated ZnS nanowire.