Annual change in the density of sulfur-oxidizing microorganisms on sandstone was enumerated to know the effects on the deterioration of stone materials of the Angkor monuments in Cambodia. Samples were obtained from total 12 stations at the Angkor Wat, Bayon, and Phnom Krom temples between 1998 and 2007. Sulfur-oxidizing microorganisms enumerated in a mineral salts medium supplemented with elemental sulfur as the sole energy source had a density of 10(1)-10(5) MPN (g sample)(-1). The sulfur-oxidizing microorganisms of the samples collected at Angkor Wat have tended to decrease in density since 2002; on the other hand, relatively constant values have been recorded in the samples of Bayon and Phnom Krom. These results suggest that the sulfur-oxidizing microorganisms on the stone play an important role in the decay of the building blocks by excreting sulfuric acid.