Rate constants and product ions were determined for a series of anions reacting with singlet molecular oxygen O-2 (a( 1)Delta(g)) at thermal energy using an electrospray ionization-selected ion flow tube. The 20 naturally occurring amino acids were used to produce corresponding deprotonated anions; only [Cys-H](-) and [Pro-H](-) were found to be reactive, with O-2 (a( 1)Delta(g)), generating OSCH2CH(NH2 )CO2- + HO and C5H6NO2- + H2O2 , respectively. The reaction O-2 (a( 1)Delta(g) ) with [Cys-H](-) has a rate constant more than ten times larger than the reaction of O-2 (a( 1)Delta(g) ) with [Pro-H](-). Furthermore, reactions of O-2 (a( 1)Delta(g) ) with carboxylic acid and thiol anions were carried out to elucidate the reactivity of the sulfur-containing functional groups. Potential energy surfaces and overall reaction exothermicities were calculated for representative reactions using density functional theory. Reactions in which attack occurs at the sulfur produce HCSO- as an ionic product. Reactions of several carboxylic acid anions likely proceed through a hydroperoxide intermediate that is analogous to that calculated for reactions with amino acid anions at a higher collision energy. Overall, rate constants for reactions of carboxylic acid anions RC(O)O- were found to be smaller for larger R groups.