The effect of hydration on the reactivity of O{sup {minus}}(H{sub 2}O){sub n} (n = 0-2) with the neutrals O{sub 2}, CO{sub 2}, CO, NO, SO{sub 2} CH{sub 4}, N{sub 2}O, and H{sub 2}O has been investigated as a function of temperature. Rate constants for the above reactions were measured at 251, 343, and 473 K ({sup {minus}}(H{sub 2}O){sub 2} reactions were studied at 251 and 343 K only) by using a variable-temperature fast-flow system that incorporates optionally either a selected ion source or a high-pressure ion source. Hydration of O{sup {minus}} was found to decrease the reactivity for the neutrals CO, SO{sub 2}, CH{sub 4} and N{sub 2}O. For reactions of O{sup {minus}}(H{sub 2}O){sub n} with O{sub 2} and CO{sub 2}, no reaction was observed for n = 0, but hydration of O{sup {minus}} enabled reaction to proceed for O{sup {minus}}(H{sub 2}O){sub 1,2}. For reaction with NO, addition of one H{sub 2}O ligand to O{sup {minus}} increased the reactivity, while addition of a second H{sub 2}O caused a decrease in the rate constant to a value below that of the unsolvated O{sup {minus}}. No reaction was observed for any of the ions O{sup {minus}}(H{sub 2}O){sub 0-2} with H{sub 2}O. Temperaturemore » dependences of the rate constants were found to vary depending on the system.« less