The 3.531 eV negative ion photoelectron spectra of the hydroperoxide ion and the tert-butylperoxide ion have been studied. We find HO2-+(h) over bar omega(351.1 nm)-->HO2+e(-) EA[HO2,(X) over tilde (2)A "]=1.089+/-0.006 eV, (CH3)(3)COO-+(h) over bar omega(351.1 nm)-->(CH3)(3)COO+e(-) EA[(CH3)(3)COO,(X) over tilde (2)A "]=1.196+/-0.011 eV. The photoelectron spectra show detachment to the ground state of the peroxyl radicals and to a low lying electronic state. The intercombination gaps are measured to be Delta E((X) over tilde (2)A "-(A) over tilde (2)A＇)[HO2]=0.871+/-0.007 eV and Delta E((X) over tilde (2)A "-(2)A＇)[(CH3)(3)COO]=0.967+/-0.011 eV. The gas phase acidity of (CH3)(3)COOH was measured in a tandem flowing afterglow-selected ion flow tube (FA-SIFT) to be Delta(acid)G(298)=363.2+/-2.0 kcal mol(-1) and we find Delta(acid)H(298)[(CH3)(3)COO-H]=370.9+/-2.0 kcal mol(-1). Use of Delta(acid)H(298)[(CH3)(3)COO-H] and EA[(CH3)(3)COO] leads to the bond energies DH298[(CH3)(3)COO-H]=85+/-2 kcal mol(-1) and D-0[(CH3)(3)COO-H]=83+/-2 kcal mol(-1). The thermochemistry of the alkylperoxyl radicals, RO2, is reviewed. A mechanism for the rearrangement of chemically activated peroxyl radicals is proposed [RO2](X) over tilde (2)A "-->[RO2]*(A) over tilde (2)A＇-->aldehydes/ketones+HO((2)Pi), [RO2](X) over tilde 2A "-->[RO2]*(A) over tilde 2A＇ -->alkenes+HO2((X) over tilde (2)A ").