The aim of this paper is to determine why, and in what manner, brick decays. Samples of brick and salt efflorescence were taken from a church in northern Spain and analysed for mineralogical and structural changes, and soluble salts. The effects of decay on brick mineralogy and texture were studied using polarizing microscopy and scanning electron microscopy (SEM) provided with energy dispersive X-ray analysis (EDXA); salts were identified by X-ray diffraction (XRD). Physical tests were used to assess the susceptibility of bricks to decay by moisture, freeze-thaw cycling and salt weathering. Mortars were examined to determine if they contributed to salt decay process in the brick. The paper concludes that, for the samples investigated, salt weathering predominated. Many of these salts, which include halite (NaCl), niter (KNO3) and gypsum (CaSO4), as well as aluminium salts, were transported from the ground into the wall by means of capillary action.