Concrete structures deteriorate under acid attack, but the mechanism of deterioration is not yet clearly understood. In this study, cement paste and mortar were immersed into an acidic solution for certain periods, then X-ray diffraction, energy dispersion X-ray analysis and EPMA analysis were carried out. It was found that Fe, Al and Mg condensed layers were produced by the attack of strong acid, while an Fe layer was not formed when a weak acid was applied. On the other hand, such layers were not formed if only carbonation was a cause of the deterioration. It has become possible to estimate the strength of attacking acid.