Iron(III)-catalyzed carbonyl-olefin ring-closing metathesis represents a new approach toward the assembly of molecules traditionally generated by olefin-olefin metathesis or olefination. Herein, we report detailed synthetic, spectroscopic, kinetic, and computational studies to determine the mechanistic features imparted by iron(III), substrate, and temperature to the catalytic cycle. These data are consistent with an iron(III)-mediated asynchronous, concerted [2+2]-cycloaddition to form an intermediate oxetane as the turnover-limiting step. Fragmentation,of the oxetane via Lewis acid-activation results in the formation of five- and six-membered unsaturated carbocycles.