Rate coefficients for the reaction HO2 + NO2 + N-2 --> HO2NO2 + N2 (reaction 1) were measured using simultaneous near-IR and UV spectroscopy from 220 to 298 K and from 45 to 200 Toff. Using the data acquired in the present experiment, the low-pressure and high-pressure limit rate constants for reaction I were determined to be k(o) = (2.1 +/- 0.1) x 10(-31) x (T/3000)(-(3.1+/-10.1)) cm(6) molecule(-2) s(-1) and K-infinity = (2.4 +/- 0.1) x 10(-12) x (T/300))(-(1.9+/-0.5)) cm(3) molecule(-1) s(-1), using the expressions for rate constants adopted by the NASA data evaluation panel (F-c = 0.6). The reaction rate was significantly enhanced in the presence of methanol due to a chaperone effect involving an HO2CH3OH complex. Enhancement parameters for this process were quantified as a function of temperature. During the course of our studies, we observed an unexpected time-dependent UV absorption unaccounted for in previous examinations of reaction I that employed UV spectroscopy to monitor HO2. We show that this absorption, which may have led to errors in those prior studies, is due to the process NO2 + NO2 reversible arrow N2O4 (reaction 3). Using UV-visible spectroscopy, we determine k(-3) to be (36 +/- 10) s(-1) at 231 K and 100 Torr using the NASA-recommended equilibrium constant for the dimerization of NO2. This represents the first measurement of k(-3) at T < 250 K.