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The short-lived positron-emitter carbon-11 (<i>t</i>_1/2 = 20.4 min; β⁺, 99.8%) is prominent for labeling tracers for use in biomedical research with positron emission tomography (PET). Carbon-11 is produced for this purpose with a cyclotron, nowadays almost exclusively by the ¹⁴N(p,α)¹¹C nuclear reaction, either on nitrogen containing a low concentration of oxygen (0.1–0.5%) or hydrogen (~5%) to produce [¹¹C]carbon dioxide or [¹¹C]methane, respectively. These primary radioactive products can be produced in high yields and with high molar activities. However, only [¹¹C]carbon dioxide has some utility for directly labeling PET tracers. Primary products are required to be converted rapidly and efficiently into secondary labeling synthons to provide versatile radiochemistry for labeling diverse tracer chemotypes at molecular positions of choice. This review surveys known gas phase transformations of carbon-11 and summarizes the important roles that many of these transformations now play for producing a broad range of labeling synthons in carbon-11 chemistry.