Ash (Fraxinus spp.) logs, infested with fully developed, cold-acclimated larval and prepupal emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), were fumigated with methyl bromide (MeBr) at 4.4 and 10.0°C for 24 h. Concentrations × time dosages of MeBr obtained were 1,579 and 1,273 g-h/m³ (24-h exposure) at 4.4 and 10.0°C after applied doses of 112 and 96 g/m³, respectively. MeBr concentrations were simultaneously measured with a ContainIR infrared monitor and Fumiscope thermal conductivity meter calibrated for MeBr to measure the effect of CO₂ on Fumiscope concentration readings compared with the infrared (IR) instrument. The presence of CO₂ caused false high MeBr readings. With the thermal conductivity meter, CO₂ measured 11.36 g/m³ MeBr per 1% CO₂ in clean air, whereas the gas-specific infrared ContainIR instrument measured 9.55% CO₂ as 4.2 g/m3 MeBr (0.44 g/m3 per 1% CO₂). The IR instrument was 0.4% as sensitive to CO₂ as the thermal conductivity meter. After aeration, fumigated and control logs were held for 8 wk to capture emerging beetles. No A. planipennis adults emerged from any of the fumigated logs, whereas 262 emerged from control logs (139 and 123/m² at 4.4 and 10.0°C, respectively). An effective fumigation dose and minimum periodic MeBr concentrations are proposed. The use of a CO₂ scrubber in conjunction with nonspecific thermal conductivity instruments is necessary to more accurately measure MeBr concentrations.