Electrospray ionization mass spectrometry in the negative ion mode, ESI(-)-MS, was used to investigate the iodide-catalyzed decomposition of H2O2 in aqueous medium. ESI(-)-MS monitoring revealed the presence of an intense anion of m/z 287, suggested to be the [I-IOOH](-) anion, which was proposed to be formed in solution (and then transferred to the gas phase by the ESI process) via an interaction between iodide and the neutral and short-lived species IOOH. Evidences for the proposed structure were obtained by CID (collision-induced dissociation) experiments, which yielded exclusively a product ion of m/z 254 (I-2(center dot-)) via a peroxide radical (HOO center dot, 33 Da) loss. High level ab initio calculations revealed that the formation of the [I-IOOH](-) anion from IOOH and I- is a thermodynamically-favored process whereas its fragmentation leading to I-2(center dot-) and HOO center dot is the most favorable dissociation channel. Hence, an unprecedented mechanism for the iodide-catalyzed decomposition of H2O2 to H2O and O-2 based on the assumption of the participation of the key intermediate IOOH was proposed.