The importance of soluble salts as an agent in stone deterioration is generally recognized. Many studies are published on this subject. Nevertheless we know little about their composition, behaviour and origin; it difficult to identify them precisely. Combined microchemical, microscopical and x-ray technique give more accurate identification than chemical analysis from water extracts from weathered stone or mortar samples. The most common salts occuring in efflorescences are sulfates, nitrates, chlorides and carbonates of alkali metals and magnesium. It can be shown that sodium carbonate is also an important salt occuring in efflorecences. It produces deteriorations very similar to those produced by sodium sulfate.