A photocatalyzed ring expansion procedure for a two-carbon homologation of cyclic carbonyl compounds, towards medium-sized ring systems, is introduced. Key to success is the catalyst fac-[Ir(CF3-pmb)3] (ET=73.3 kcal mol−1), which allows for selective activation of the substoichiometric enol species. The mechanism has been corroborated by time-resolved spectroscopy and enabled a full kinetic analysis of the dynamic kinetic sensitization scenario.