Landfill-biogas utilization is a win-win solution as it creates sources of renewable energy and revenue while diminishing greenhouse gas emissions. However, the combustion of a siloxane-containing biogas produces abrasive microcrystalline silica that causes severe and expensive damages to power generation equipment. Hence, the importance of siloxane analysis of the biogas has increased with the growth of the waste-to-energy market. We have investigated an improved method for the analysis of octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) in biogas using deuterated hexamethyldisiloxane (HMDS-d(18)) as an internal standard with direct atmospheric pressure chemical ionization/tandem mass spectrometry (APCI-MS/MS). The use of HMDS-d(18) as a single internal standard provided effective signal compensation for both D4 and D5 in biogas and improved the sensitivity and reliability for the direct APCI-MS/MS quantification of these compounds in biogas. Low detection limits ( approximately 2 microg/m(3)) were achieved. The method was successfully applied for the determination of D4 and D5 contents in various samples of biogas recovered for electrical power generation from a landfill site in Montreal. Concentrations measured for D4 and D5 were in the ranges of 131-1275 and 250-6226 microg/m(3), respectively. Among the various landfill zones sampled, a clear trend of decreasing D4 and D5 concentrations was observed for older landfill materials.