The kinetics of decomposition of pernitrite in excess of alkali have been studies at several temperatures. These show that decomposition occurs through the pernitrous acid molecule, HO·ONO, with ΔH‡= 12·5 kcal. mole–1 and ΔS‡= 26 e.u. The value of ΔH‡ is not consistent with the accepted view that pernitrous acid decomposes via homolytic fission of the O–O link. This problem is discussed and other, non-homolytic, mechanisms suggested. An upper limit for the pKA of pernitrous acid at 2° of 8·3 has been calculated. The first isolation of a solid pernitrite is reported. The extinction coefficient of the pernitrite anion at 302 mµ(λmax.) has been redetermined as 1670 ± 50.