The mechanism of sulfate attack is understood, but it is often difficult to find the origin of the sulfate: soil, ground water, sea water, atmosphere, ground-fill, and de-icing salts. Stable sulfur isotope ratios provide a means of identifying the sulfur source. Experimental details and typical results using mass spectrometry are described. The primary reason for the research was the study of deterioration of roads and bridges, but bricks and building stones (architectural) were also investigated.