The geometric structure of (fluorocarbonyl)sulfenyl isocyanate, FC(O)SNCO, was determined by gas electron diffraction. Only a syn-syn conformation (carbonyl C=O syn with respect to S-N and the NCO group syn to C-S) is compatible with the experimental diffraction intensities. This structure possesses an extremely short contact between the carbonyl oxygen atom and the isocyanate carbon atom of 2.65 9 as compared to the van der Waals distance of 3.20 9. Several theoretical calculations (ab initio and density functional methods) confirm that this conformer represents the most stable structure.