Sodium sulfate is one of the most damaging and complex salts typically involved in the deterioration of our architectural heritage. One of the main difficulties is to determine which of its crystalline phases, that is, thenardite, mirabilite, or the metastable heptahydrate, will precipitate under certain conditions. Indeed, there is a significant range of temperature and relative humidity in which these phases can crystallize. Furthermore, one precipitated crystalline phase may under certain conditions transform to another one. Here, we show that nuclear magnetic resonance can be successfully used to non-destructively define the phase being precipitated. We investigate delamination of Anc¸ã limestone due to sodium sulfate crystallization, a type of decay which is representative of those occurring in real constructions. The decay was achieved during isothermal drying of stone specimens under different environmental conditions. The work allowed concluding that both mirabilite and heptahydrate can be responsible for this type of decay in different conditions. The heptahydrate tends to crystallize when there is no previous presence of mirabilite crystals in the porous material.