The N 2 and O 2 pressure broadening coefficients of the pure rotational transitions at 625.66 GHz ( N Ka Kc =10 1▒9–10 0▒10, J=10.5–10.5) and 649.70 GHz ( N Ka Kc =10 2▒9–9 2▒8, J=9.5–8.5) in the vibronic ground state X 2 A′ of the perhydroxyl (HO 2) radical have been determined by precise laboratory measurements. For the production of HO 2, the mercury-photosensitized reaction of the H 2 and O 2 precursors was used to provide an optimum condition for measurement of the pressure broadening coefficient. The Superconducting Submillimeter-wave Limb Emission Sounder (SMILES) was designed to monitor the volume mixing ratio of trace gases including HO 2 in the Earth's upper atmosphere using these transitions. The precise measurement of pressure broadening coefficient γ in terms of the half width at half maximum is required in order to retrieve the atmospheric volume mixing ratio. In this work, γ coefficients of the 625.66 GHz transition were determined for N 2 and O 2 at room temperature as γ(N 2)=4.085±0.049 MHz/Torr and γ(O 2)=2.578±0.047 MHz/Torr with 3 σ uncertainty. Similarly, the coefficients of the 649.70 GHz transition were determined as γ(N 2)=3.489±0.094 MHz/Torr and γ(O 2)=2.615±0.099 MHz/Torr. The air broadening coefficients for the 625.66 GHz and 649.70 GHz lines were estimated at γ(air)=3.769±0.067 MHz and 3.298±0.099 MHz respectively, where the uncertainty includes possible systematic errors. The newly determined coefficients are compared with previous results and we discuss the advantage of the mercury-photosensitized reaction for HO 2 generation. In comparison with those of other singlet molecules, the pressure broadening coefficients of the HO 2 radical are not much affected by the existence of an unpaired electron.