A combined mass-spectrometric and theoretical approach has been used for an investigation of the gas-phase chemistry of two representative alkylperoxy anions. Metastable CH3OO- ions undergo unimolecular loss of molecular hydrogen and formaldehyde yielding HCO2- and OH-, respectively. The observed reactivity is in pleasing agreement with calculations of the [C,H-3,O-2](-) and [C,H,O-2](-) potential-energy surfaces at the BECKE3LYP/6-311++G** level of theory. Upon exhaustive methylation of the alpha-position as in t-C4H9OO- anions, the reactivity switches completely to an elimination of (CH3)(2)C=CH2 giving rise to the formation of HOO-. The results obtained for the ''bare'' alkylperoxy anions are used for the analysis of the EI mass spectrum of (t-C4H9OOLi)(12) dodecamers, which thermally decompose in the inlet system at a probe temperature of ca. 130 degrees C. The decomposition is rationalized by a mechanism involving nucleophilic attach of one 1-C4H9OOLi subunit on the oxenoid oxygen atom of a second t-C4H9OOLi moiety. This reaction may produce t-C4H9OOOLi trioxy species as intermediates, which rapidly decompose to singlet dioxygen and t-C4H9OLi.