The complete microbial degradation of thiodiglycol (TDG), the primary hydrolysis product of sulfur mustard, by Alcaligenes xylosoxydans ssp. xylosoxydans (SH91) was accomplished in laboratory-scale stirred-tank reactors. An Andrews substrate inhibition model was used to describe the cell growth. The yield factor was not constant, but a relationship with initial substrate concentration has been developed. Using a substrate-inhibition and variable-yield kinetic model, we can describe the cell growth and substrate consumption in hatch and repeated batch fermentations. Several reactor-operating modes successfully degrade TDG concentration to below 0.5 g/L. According to the experimental results, the two-stage repeated batch operation has the best degradation efficiency, and it also can degrade 500 mM TDG (approximate to 60 g/L) to 5 mM (approximate to 0.7 g/L) in <5 d. A hypothesis for explaining : variable-yield and byproduct formation based on the capacity and utilization of metabolic loads is presented.