Sulfur oxidizing bacteria (SOB) have been found to be important promoters of concrete biodeterioration. Some microorganisms such as Halothiobacillus neapolitanus, Thiobacillus thioparus, and Acidithiobacillus thiooxidans have been associated with important weight losses in concrete structures exposed to humid and aerated environments with low to high concentrations of hydrogen sulfide and carbon dioxide. Particularly sewers and some underground structures can be exposed to these aggressive conditions and suffer an accelerated deterioration. In this paper we present the results of a set of experiments using mortar samples with high surface-to-volume ratio inoculated with SOB and exposed up to 200 ppm H2S in nonaseptic conditions. The results showed significant changes in mechanical properties such as weight, strength, and porosity in less than six months. Particularly observed weight losses are larger than those reported by previous studies in the same time span suggesting that the highly exposed surface-to-volume ratio used here is a predominant factor for the biodeterioration rate.