The unimolecular decomposition of HClO4 has been investigated at the G2M//B3LYP/6-311 + G(3df, 2p) level of theory. Above 500 K, the decomposition process was found to depend strongly on pressure below 200 atm. The rate constants at the high- and low-pressure limits were predicted to be k1 ∞ = 1.5 × 1017 exp(-26500/T) s-1 and k1 0 = 3.39 × 1030 T-10.9 exp(-29430/T) cm3 molecule-1 s -1, respectively. Under the atmospheric-pressure condition, the predicted first-order rate coefficient for the temperature range 300-3000 K, k1 = 5.0 × 1051 T-11.64 exp(-30700/T) s-1, agrees reasonably with experimental data obtained at 550-750 K by different experimental groups. We have also calculated the rate constants for the bimolecular reaction of OH with ClO3 producing HClO4 by association/stabilization and HO2 + ClO2 by association/fragmentation. At the high-pressure limit, the rate constants can be given as:k-1 = 3.2 × 10-10 T0.07 exp(-25/T) cm3 molecule-1 s-1 and k2 = 2.1 × 10-10 T0.09 exp(-18/T) cm3 molecule-1 s-1, respectively, for the temperature range 300-3000 K. Under the atmospheric pressure condition, the OH + ClO3 reaction produces predominately the HO2 + ClO2 products.