The reaction of iron sulfate with 1-carboxymethyl-4,7-dimethyl-1,4,7-triazacyclononane (L) and hydrogen peroxide in aqueous ethanol gives a brown dinuclear complex considered to be [Fe2(N3O-L)2(mu-O)(mu-OOCCH3)] + (1), which converts upon standing in acetonitrile solution into the green tetranuclear complex [Fe4(N3O2-L)4(mu-O)2]4+ (2). A single-crystal X-ray structure analysis of [2][PF6]4.5MeCN reveals 2 to contain four iron(III) centers, each of which is coordinated to three nitrogen atoms of a triazacyclononane ligand and is bridged by one oxo and two carboxylato bridges, a structural feature known from the active center of methane monooxygenase. Accordingly, complex 2 was found to catalyze the oxidative functionalization of methane with hydrogen peroxide in aqueous solution to give methanol, methyl hydroperoxide, and formic acid; the total turnover numbers attain 24 catalytic cycles within 4 h. To gain more insight into the catalytic process, the catalytic potential of 2 was also studied for the oxidation of higher alkanes, cycloalkanes, and isopropanol in acetonitrile, as well as in aqueous solution. The bond selectivities of the oxidation of linear and branched alkanes suggest a ferroxy radical pathway.