The recombination reaction between IO and NO 2 was studied over a large range of temperature (216-474 K) and pressure (20-760 Torr). IO, produced by the pulsed photolysis at 193 nm of NO 2 to yield O in the presence of CF 3 I, was probed at 445.0 nm [IO(A 2 Π 3 / 2 - X 2 Π 3 / 2 ), (2,0)] and monitored by nonresonant LIF at 458.6 nm [IO(A 2 Π 3 / 2 - X 2 II 3 / 2 ). (2,1)]. The resulting rate coefficients were then fitted by RRKM theory, using molecular parameters and a heat of formation for INO 3 (ΔH° f , 0 = 70 ′ 16 kJ mol - 1 , best fit value 80 kJ mol - 1 ) derived from quantum calculations at the B3LYP/6-31 1+g(2d,p) level. This yielded k r e c , 0 = (1.3 ′ 0.2) × 10 - 3 0 (T/300 K) - ( 4 . 5 γ 0 . 6 ) cm 6 molecule - 2 s - 1 , k r e c , ∞ = (6.5 ′ 1.0) x 10 - 1 2 (T/300 K) - ( 1 . 3 γ 0 . 8 ) cm 3 molecule - 1 s - 1 , and F c = 0.57, over the experimental range of temperature and pressure. The RRKM calculations, in conjunction with measurements made at 474 K, were used to determine the rate of thermal dissociation of INO 3 for the conditions of the lower atmosphere: k d i s s (240-305 K, 760 Torr) = 1.1 x 10 1 5 exp(-12060/T s - 1 , with an upper limit about a factor of 2 higher. The atmospheric significance of these results is discussed briefly.