Odorant concentrations and the pH distribution in a carrier-packed biological deodorization (CPBD) reactor which is currently being used in a sewage treatment plant were measured. The results showed that more than 99% of the H2S and 70-80% of the MT were removed from the raw gas in the early section of the packed bed, which comprised one quarter of the total volume of the CPBD reactor. For this reason, this section of the packed bed was considered to be essential in the removal of sulfur odorants. The pH in this section was very low (pH 1-2), and changed little even after water sprinkling. Thiobacillus thiooxidans KS1 was the dominant strain isolated from the early section of the reactor. Kinetic analysis of H2S removal by KS1 was carried out using a closed stirred-tank reactor and considering the physical absorption rate of H2S. The effects of pH and temperature on the maximum specific H2S removal rate (V-m = mu mol . min(-1). cell(-1)) were determined. Since strain KS1 is classified as an acidophilic sulfur-oxidizing bacterium, the V-m as measured in the acidic pH range. At 30 degrees C, the V-m at pHs 2.5 and 1.0 was 2.84 x 10(-10) and 2.29 x 10-(10)mu mol . min(-1). cell(-1), respectively. On the other hand, at 10 degrees C, pH 2.5, the V-m was 0.12 x 10(-10)mu mol min(-1). cell(-1), which was only about 1/20 the removal rate at 30 degrees C.