Isotopically labeled formaldehyde (*C§H2O) is prepared from labeled methyl iodide (*C§H3I) by reaction with an oxygen nucleophile having a pendant leaving group. The mild and efficient reaction conditions result in good yields of *C§H2O with little or no *C isotopic dilution. The simple, efficient production of 11CH2O is described. The use of the 11CH2O for the formation of positron emission tomography tracer compounds is described. The reaction can be incorporated into automated equipment available to radiochemistry laboratories. The isotopically labeled formaldehyde can be used in a variety of reactions to provide radiotracer compounds for imaging studies as well as for scintillation counting and autoradiography.