It was observed by X-ray single crystal analysis that trans-2-buten-1-yl cobaloxime complex with pyridine or 4-bromopyridine as a axial ligand isomerizes to the cis isomer with retaining the single crystal form on exposure to a Xe lamp. The complex with 3-chloropyridine or methylsulfide as a axial ligand did not show the isomerization. The reaction cavities for the trans-2-butenyl groups calculated from four crystal structures showed that those of reactive crystals are significantly larger than those of non-reactive ones.