This arrangement is feasible as the quantities of H2S increase and for systems operating with sufficient working pressure (about one-third psig or more). Two vessels are arranged in series, flowing gas through the first bed into the second. The lead vessel acts as the “working” unit to remove all of the H2S at the beginning of a treatment period with its outlet H2S increasing over time. The gas can go to the second, or “lag” vessel for further polishing, or it can bypass the second vessel up to the desired gas H2S outlet specification. The operation of the lead bed is similar to the single-vessel arrangement. The second vessel can be placed in operation at startup, or it can be used to polish the H2S remaining in the gas from the lead vessel when the level of outlet H2S reaches the maximum specification. Once the lead vessel inlet and outlet concentration are equal, the material is considered spent or exhausted. The gas flow is then directed to the second vessel, while the spent material is removed from the lead unit. The second vessel becomes the lead or “working” unit with the new product bed (in the lead unit) operating as the lag or polishing unit — all without gas-flow interruption. The lead/ lag arrangement allows more efficient use of the granular material with no interruption in unit service and greater process reliability. When operating in this lead/lag mode, removal of the spent material can be conveniently scheduled using the days or weeks allowed during the change-out “window” — without exceeding maximum outlet H2S concentrations.