The strain-rate cycling test during the Blaha effect measurement at 77-254 K was carried out for four kinds of single crystals : KCl doped with Mg2+, Ca2+, Sr2+ or Ba2+ as a weak obstacle. it was found that the critical temperature, at which the effective stress is zero, increases when the divalent ionic size approaches increasingly that of the K+ ion. Furthermore, the activation energy for the interaction between a dislocation and the divalent ion-vacancy dipole was determined. The activation energy increases with the divalent ionic size. Fleischer’s model attributes this to the difference of tetragonality around the dipole for each specimen.