The yield stress and the activation volume for Zr-Sn alloys with 0.74, 2.85, 4.27 and 6.19 wt% Sn have been measured at temperatures between 400 and 750 K. The temperature dependence of the yield stress exhibits a plateau except for the alloy with the highest conte nt of tin. The yield stress increases with increasing content of tin atoms. A non-monotonic variation of the activation volume with temperature has been observed for pure zirconium and for Zr-Sn alloys with 0.74 and 2.85 wt% Sn. The maximum value of the activation volume (at about 600 K) decreases with increasing content of tin. Dynamic strain ageing is considered to be responsible for the maximum in the temperature dependence of the activation volume. The dislocation structure has been observed. The experimental results are interpreted in terms of a simple model which considers that the flow stress is determined by thermally activated glide of dislocations through obstacles, dynamic strain ageing and a strengthening effect of tin atoms.