Abstract11C‐Labelled benzene derivatives were prepared catalytically by trimerizing acetylene‐11C with acetylene and propyne carriers over 1.5 grams of silica‐alumina support activated by potassium chromate (0.2% by weight). Optimized yields of the11C‐labelled aromatics were obtained when the reactants were exposed to the catalyst for 10 minutes at 24°C. A minimum addition of 50 μmole of alkyne carrier was necessary before the trimerization proceeded to give appreciable yields of11C‐labelled aromatics. In the competitive trimerization of acetylene‐11C with acetylene and propyne carriers, three11C‐labelled aromatic products were generated: benzene, toluene, and xylene. From these studies acetylene was observed to be 1.6 times more reactive than propyne. In addition, the observed distribution of11C‐labelled xylenes (3.4% ortho, 10.6% meta, and 86% para) indicates an apparent regiospecificity in the trimerization of11C‐ace