While the removal of odors emitted from various locations in sewage collection systems by pumping the gases through a soil filter has been well documented, the actual removal mechanisms remained to be established. The experiments performed involved variation of the average particle size and moisture content of the filters, and further experiments were also performed with sterile soil. It was determined that the soil filter was particularly effective in the removal of polar gases such as hydrogen sulfide and methyl mercaptan from waste streams. At hydrogen sulfide concentrations in excess of 100 mg/l the biological removal mechanisms were not significantly operative, and chemical removal mechanisms appeared to prevail. In dry sterile soil, adsorption coupled with surface catalysis reactions were found to be the predominant mechanisms for removal. For low gas concentrations (less than 100 mg/l H2S) the biological removal mechanisms were predominant and wet soil was preferred. Moist loam soil was able to remove 100% of a 775 mg/l mercaptan, in quantities of .85 liter per cu. ft. of soil/week. Methane was not removed in any case. With proper design and care, soil filters operated in a range favorable for the growth of microorganisms can be operated indefinitely.