The e.s.r. spectrum of the t-butoxycarbonyl σ-radical, generated by u.v. irradiation of a liquid mixture of di-t-butyl peroxide and t-butyl formate, has been observed. Decarboxylation of this radical has been monitored by kinetic e.s.r. spectroscopy and the unimolecular rate constant for the reaction But·ĊO → But·+ CO2 is given by log (k/s–1)= 13·4 – 12·1/θ where θ= 2·303 RT/kcal mol–1. These kinetic parameters differ from those reported in an earlier communication because it was assumed that the rate constants for reaction of a t-butyl radical with a t-butyl radical or with a t-butoxycarbonyl radical had the same value. This assumption has been tested experimentally and shown to be incorrect. Abstraction of hydrogen from the alkoxy-groups of alkyl carbonates gives radicals which do not decompose to form alkoxycarbonyl radicals.