Technical Abstract: The interactions between temperature and moisture that regulate seed aging were measured using rye seeds. Experiments include a number of long term storage studies under conditions of varying relative humidity (RH), water content and temperature. Decrease in germination percentage and radicle length of germinated seedlings with storage time was modeled using Avrami kinetics and longevity expressed as the time for a 50% reduction (P50). Seed deterioration was always slower at lower storage temperatures and significantly faster deterioration was observed in seeds stored at < 5% RH. Maximum P50s were observed between 5-6, 8-13, and 8-20% RH at 45, 35 and 25C, respectively, which is reflected by an increasing optimum water content for storage with decreasing storage temperature. Aging rate followed Arrhenius behavior, but Arrhenius parameters varied by storage RH. The profound interaction between temperature and moisture that controls seed aging is not consistent with classic fragile glass models. It may be possible to use Arrhenius relationships to predict longevity of rye seeds at low storage temperatures and RH. We postulate that intraspecific variation of seed longevity corresponds to the pre-exponential factor of the Arrhenius equation, and this may provide a fundamental insight on the mechanisms of seed aging and the differences in seeds’ responses to changing water contents.