In Earth’s troposphere, the reaction of OH and NO2 is a critical reaction in the formation of smog, and as a reservoir for HOx and NOx. While initially assumed that the only product is nitric acid (HONO2), it has since been demonstrated that a second channel exists, leading to the production of peroxynitrous acid (HOONO) in small amounts. The yields of HOONO and HONO2 are important parameters for atmospheric models, but vary as a function of temperature and pressure. Our group has previously used mid-infrared cavity ringdown spectroscopy to measure the OH stretches of HOONO and HONO2 to determine the branching ratio. Here, we present the first measurements of the branching ratio as a function of atmospherically relevant temperatures (254 – 333 K) and pressures (50 – 700 torr).