Abstract New yield-stress data have been obtained for NaC1:Pb and NaCI:Cd crystals and correlated with well defined precipitation stages of the impurities as monitored by optical spectroscopy techniques (absorption, luminescence and Raman). For NaCl:Pb, the hardening induced by both metastable and PbCl2, precipitates is athermal in accordance with an Orowan-type mechanism. For NaCl:Cd, the correlation between the formation of the Suzuki-phase precipitates and the softening of the crystal has been quantitatively established. From our experiments the yield-stress associated with the dipoles forming the Suzuki-phase precipitates has been found to be ≃ ½ of that corresponding to the isolated dipoles. The hardening induced by these precipitates shows a marked temperature dependence, which appears to be consistent with a dislocation-cutting mechanism.