The effect of the addition of some ten ppm sulfur to α-zirconium and Zr-l%Nb-0.13%O alloy is investigated. A strong hardening effect is demonstrated. The hardening effect appears at about 100°C and is maximal around 400°C. Based on creep and constant strain-rate tests, and scanning electron microscopy and transmission electron microscopy observations, it is concluded that sulfur modifies the deformation kinetics, with no change in the deformation mechanisms. The sulfur effect is viscous-like. The screening of all the potential controlling mechanisms allows us to postulate a sulfur segregation involving modifications of the dislocation core structure and then of the dislocation mobility. Ab initio simulations are consistent with this hypothesis.