The reaction of OH with NO2 is pivotal in both stratospheric and tropospheric chemistry; in each case it is the dominant homogeneous mechanism for conversion of NOx to NOy. The rate constant is a strong function of pressure and temperature, and key portions of the pressure-temperature domain are poorly or ambiguously covered by the available data. These include conditions typical of the tropospheric boundary layer and of the lower stratosphere. At surface conditions differences of 60% exist both in the literature data and between the major recommendations, while at lower stratospheric conditions there are few available data. Our High Pressure Flow kinetics system is ideally suited to studying this reaction, as we are able to scan both temperature and pressure while maintaining wall-less conditions, eliminating the possible complications of heterogeneous chemistry. Here we report a temperature-and pressure-dependent study (220--300 K, 50--150 torr) of this reaction; the measured rate constants are in excellent agreement with previously published values down to 240 K, but lie 10--20% lower than the historical data available below that temperature. An analysis of all available data motivates a large (~20%) downward revision in the recommended rate constant below room temperature. ¿ 1999 American Geophysical Union