Fourteen mature technology biomass refining scenarios – involving both biological and thermochemical processing with production of fuels, power, and/or animal feed protein – are compared with respect to process efficiency, environmental impact – including petroleum use, greenhouse gas (GHG) emissions, and water use–and economic profitability. The emissions analysis does not account for carbon sinks (e.g., soil carbon sequestration) or sources (e.g., forest conversion) resulting from land-use considerations. Sensitivity of the scenarios to fuel and electricity price, feedstock cost, and capital structure is also evaluated. The thermochemical scenario producing only power achieves a process efficiency of 49% (energy out as power as a percentage of feedstock energy in), 1359 kg CO2 equivalent avoided GHG emissions per Mg feedstock (current power mix basis) and a cost of $0.0575/kWh ($16/GJ), at a scale of 4535 dry Mg feedstock/day, 12% internal rate of return, 35% debt fraction, and 7% loan rate. Thermochemical scenarios producing fuels and power realize efficiencies between 55 and 64%, avoided GHG emissions between 1000 and 1179 kg/dry Mg, and costs between $0.36 and $0.57 per liter gasoline equivalent ($1.37 – $2.16 per gallon) at the same scale and financial structure. Scenarios involving biological production of ethanol with thermochemical production of fuels and/or power result in efficiencies ranging from 61 to 80%, avoided GHG emissions from 965 to 1,258 kg/dry Mg, and costs from $0.25 to $0.33 per liter gasoline equivalent ($0.96 to $1.24/gallon). Most of the biofuel scenarios offer comparable, if not lower, costs and much reduced GHG emissions (>90%) compared to petroleum-derived fuels. Scenarios producing biofuels result in GHG displacements that are comparable to those dedicated to power production (e.g., >825 kg CO2 equivalent/dry Mg biomass), especially when a future power mix less dependent upon fossil fuel is assumed. Scenarios integrating biological and thermochemical processing enable waste heat from the thermochemical process to power the biological process, resulting in higher overall process efficiencies than would otherwise be realized – efficiencies on par with petroleum-based fuels in several cases. © 2009 Society of Chemical Industry and John Wiley & Sons, Ltd