Most theories concerning the deterioration of porous building materials by the action of salts are based on the assumption of pressures exerted by crystallizing and, moreover, hydrating salts. Salt crystallization tests, which exist in various standardized forms, are mostly based on the hydration pressure, building up by cycles of soaking with salt solution followed by drying and heating and so on. However, theoretical considerations as well as simple laboratory experiments lead to the conclusion that hydration of salt anhydrates, namely by action of concentration solutions, is a process unlikely to occur in the course of laboratory tests. On the other hand, the formation of anhydrates from hydrates always proves to result in very fine-grained, even amorphous aggregates. Their high specific surface and microporosity makes them susceptible to thermohygric dilatation and structural alterations in relative humidity and temperature. Hydration of such anhydrate aggregates, however, will be very limited. Based on the above considerations, alternative test cycles are discussed and examples presented.