The lifetime of Hg(6³P₀) in a gas mixture of Ar + N₂ at room temperature was measured by a phase-shift method. A mixture containing a trace of Hg vapour was illuminated by 253.7 nm radiation, whose intensity was modulated at a frequency in the range of 120–1000 Hz, and the resulting a.c. component of Hg(6³P₀) concentration was monitored by a lock-in system. The lifetime is inversely proportional to the square root of the 253.7 nm radiation intensity. This is attributable to accumulation of N₂ in the v= 1 level through the spin-orbit relaxation Hg(6³P₁)+ N₂(v= 0)→ Hg(6³P₀)+ N₂(v= 1), and the reverse of this reaction results in shortening of the lifetime. The accumulation of N₂ in the v= 1 level is due to the fact that N₂(v= 1) is deactivated by a factor of about 10² less efficiently than Hg(6³P₀). The observed dependence of the lifetime on the 253.7 nm intensity made it possible to deduce the rate constant of the relaxation. The result agreed with the value determined from the lifetime measurement of Hg(6³P₁) in N₂. Thus, it is concluded that the spin-orbit relaxation of Hg(6³P₁) in N₂ consists of the electronic-to-vibrational energy transfer. The reciprocal lifetime of N₂(v= 1) was determined to be 1.2 ± 0.3 s^–1 in the mixture of 1 Torr N₂+ 9 Torr Ar. This leads to the estimate that the collision probability for deactivation of N₂(v= 1) on the wall of a quartz cell is 1 × 10^–4.