Distributed generation (DG) offers a number of potential benefits, but questions remain about environmental performance. Air emissions from five key DG technologies; gas engines, diesel engines, gas turbines, micro-turbines, and fuel cells, were systematically compared with total energy supply systems based on centralized gas turbines (CCGT) and coal steam turbines plus distributed heating (DH) using gas-fired boilers. Based on emissions and operational factors from existing commercially marketed DG-CHP technologies, combined heat and power (CHP) applications are considered, which are remotely monitored and operated as base-load supply. Emissions results are characterized using heat-to-power ratios (HPRs), which concisely describe different types of energy demand under different applications or seasonal conditions. At an HPR of zero (i.e. the special case of electricity-only), CCGT with DH gives the lowest emissions portfolio, but at HPR values typical for buildings in the United States, efficiency advantages ensure gas-fired combustion DG-CHP technologies become broadly competitive across the range of key emissions. Fuel cell DG-CHP provides a very low emissions portfolio, but at a significant cost premium. At higher HPR values, emissions from heat supply can become a key issue, leading to the surprising finding that some combustion-based DG-CHP systems have lower total emissions than fuel cell-based systems. Based on these insights, the paper concludes with a discussion of streamlined yet rigorous regulatory approaches for DG-CHP technologies. (c) 2005 Elsevier Ltd. All rights reserved.