The SO3:CO heterodimer has been found by ab initio calculations to form a complex in which the C lone pair of CO interacts with the pi*(SO) antibond via the pi-hole lying directly above the S atom of SO3. The binding energy of this complex is 4.3 kcal/mol, with Coulombic attraction as its main component. There is also a secondary minimum, with half that strength, wherein the CO molecule is rotated so that it is its O atom that interacts with SO3. The most stable SO3:(CO)(2) heterotrimer has the two CO molecules approaching the S atom from above and below the SO3 plane with the C atoms of the CO interacting with the S of the SO3. A strong chalcogen bond between SO3 molecules is the dominant feature of the (SO3)(2):CO trimer, supplemented by a S center dot center dot center dot C chalcogen bond in the SO3:CO dimer.