The decarboxylation of tert-butyl-9-methylfluorene-9-percarboxylate is investigated by picosecond UV pump/IR probe spectroscopy, An intense 308 nm UV pulse of 1 ps duration is used for excitation. The formation of CO2 is monitored on a picosecond time scale via the absorption of the asymmetric stretch at 2335 cm(-1). The mode of bond scission, concerted or stepwise, is discussed.