Gas　permeability　(ka)　and　gas　diffusion　coefficient　(Dp)　are　vital　for　controlling　gas　transport　through　earthen　landfill　cover　overlying　municipal　solid　wastes　or　acid　mine　tailings.　Soil　clods　generated　during　the　construction　of　landfill　cover　can　result　in　various　soil　microstructures.　However,　the　effects　of　soil　microstructure　on　Dp　and　ka　are　unclear.　This　study　aimed　to　investigate　and　compare　effects　of　soil　microstructure　on　measured　ka　and　Dp　of　compacted　manufactured　sand　tailings　(MST)-bentonite　mixtures,　considering　the　effects　of　specimen　preparation　method,　compaction　water　content　(wcomp),　and　wetting　path.　The　related　pore　size　distributions　(PSDs)　were　measured　by　mercury　intrusion　porosimetry.　The　measurements　show　that　the　average　ka　of　as-compacted　specimens　increased　and　then　decreased　as　wcomp　increased　(i.e.,　a　lower　soil　air　content　ϵ　-　gas-filled　volume　per　unit　soil　volume),　while　Dp　reduced　monotonically.　It　was　because　the　compacted　MST-bentonite　mixtures　exhibited　a　multimodal　PSD,　in　which　macropores　significantly　increased　the　average　advective　flow　velocity　of　gas.　However,　the　average　diffusive　flow　velocity　of　gas　was　independent　of　pore　size,　according　to　the　kinetic　theory　of　gas　molecules.　Conversely,　ka　and　Dp　both　decreased　as　ϵ　decreased　when　the　water　content　of　the　as-compacted　specimen　was　increased　along　the　wetting　path　by　spraying　water,　due　to　the　wetting-induced　collapse　of　macropores.　These　illustrated　that　Dp　was　mainly　controlled　by　the　ϵ　and　tortuosity　of　gas-filled　pores,　while　ka　was　dominated　by　well-connected　gas-filled　macropores.　Moreover,　the　PSD　of　specimens　was　affected　by　the　specimen　preparation　method,　bentonite　content,　and　sieving　treatment,　thus　affecting　ka　and　Dp.　　©　2023　American　Society　of　Civil　Engineers.