Here we present a protocol for labeling aliphatic carboxylic acids with the positron-emitting radionuclide 11C (t(1/2) = 20.4 min) at the carboxyl position using [11C]carbon monoxide via photoinitiated free radical-mediated carbonylation. A solution of an alkyl iodide in a homogenous binary organic solvent-water mixture is introduced into a high-pressure photochemical reactor containing [11C]carbon monoxide. Then the reactor contents are pressurized to 40 MPa and irradiated with ultraviolet light for 6 min. The labeled product is purified using HPLC. All manipulations with radioactivity including the labeling synthesis are carried out on an automated Synthia system. In a typical case, 3.19 GBq of purified [1-(11)C]1,10-decanedicarboxylic acid (with a specific radioactivity of 188 GBq/micromol) can be obtained within 35 min after the end of a 10-microAh bombardment. Compared to previous labeling methods, this protocol is compatible with a wider range of functional groups, utilizes less-sensitive precursors, and is less subject to isotopic dilution.