As part of a program to investigate radiation damage in geological materials of interest to the radioactive waste disposal program, radiation damage—particularly radiation-induced sodium metal colloid formation—has been studied in 14 natural rock salt samples. All measurements were made with equipment for making optical absorption and other measurements on samples, in a temperature-control irradiation chamber, during and after 1.5-MeV electron irradiation. Samples were chosen for practical and scientific purposes from localities that are potential repository sites and from different horizons at certain localities. At room temperature and at low doses, only F-centers (Cl− ion vacancies) and a variety of “V-region”absorption bands (mostly holes trapped on a variety of defects) are present. Above 100°C and at high doses, intense sodium metal colloid particle absorption bands are formed. Formation of F-centers begins immediately after the irradiation is initiated and increases monotoni-cally to a well-defined plateau, reached at doses of 106 to 107 rad. In natural rock salt samples, not purposely strained in the laboratory, the colloid formation is well described by classical nucleation and growth curves with the initial induction, or low growth rate nucleation region, extending to 106 to 107 rad. In all samples measured to date, the rapid colloid growth region is well described by C(irradiation time)n or C(dose)n relations. Aside from the temperature dependence, at least three other factors modify the colloid growth rate: 1. The induction period is shortened by straining samples prior to irradiation. For strains up to ∼10%, it is progressively shortened. Around 10% strain, the induction period is negligible or zero. Strains above this value do not produce large additional effects. 2. The colloid formation rate increases as the dose rate decreases, i.e., on a unit dose basis, lower dose rates produce more colloids than higher dose rates. 3. Colloid formation appears to be related to the salt impurity level It is suppressed in regions of crystals containing ∼1% calcium and sulfur. Both the F-center and colloid particle formation is strongly temperature dependent. At fixed irradiation conditions, the F-center plateau level is high at 100°C and decreases monotonically to a low or negligible level at 300°C. The colloid formation rate is low or negligible at irradiation temperatures of 100 to 115°C and, as the radiation temperature is increased, increases to a broad maximum at 150 to 175°C and then decreases to a negligible level at 250 to 300°C. The colloid growth curve induction period is >104 s at 100°C, decreases to <3000 s at 150 to 175°C, and is >104 s at 275 to 300°C. Colloid formation has been carefully studied in the 14 “naturally impure” rock salt samples at a temperature where the colloid formation rate is a maximum—namely 150°C at a dose rate of 1.2 × 108 rad/h and to total doses of 2 to 4 × 108 rad. Colloid formation rates vary by a factor of 10 or more between samples from different localities. Using the C(dose)n relation to estimate the colloid formed in actual repositories indicates that in 50 to 400 yr, a dose of 1010 rad will convert between 0.1 and 10%, and 2 × 1010 rad will convert between 1 and 50% of the salt to colloidal sodium metal. The chemical and physical properties of rock salt containing colloid sodium metal at these levels differs markedly from that of normal rock salt, and irradiated salt is likely to interact vigorously with canisters, backfill materials, water, etc.