The ultrafast photodissociation of tert-butyl-9-methylfluorene-9-percarboxylate (TBFC) is studied by mid-infrared transient absorption spectroscopy after UV excitation at 266 nm. By means of ¹³C-labeled TBFC and additional DFT calculations transient IR bands in the fingerprint region were unambiguously assigned to the methylfluorenyl radical. The experiments show that the fragmentation is controlled by the S₁-lifetime of TBFC and, dependent on the solvent, within 0.8–2.1 ps leads to tert-butyloxy and methylfluorenyl radicals plus CO₂via concerted bond breakage of the O–O and the fluorenyl–C(carbonyl) bond. In accordance, the CO₂ quantum yield is determined to be unity.