Ion/molecule reactions in octafluorocyclobutane (c-C4F8) were studied using a high-pressure mass spectrometer. The thermochemical stabilities of the cluster ions of halide ions (X-) with c-C4F8 were measured. While the F- ion forms a covalent bond with c-C4F8, the interaction between other halide ions with c-C4F8 is mainly electrostatic. Theoretical calculation revealed that the halide ions interact not with the lowest unoccupied molecular orbital but with the next lowest unoccupied molecular orbital of the c-C4F8 molecule in the most stable cluster ions X-(c-C4F8). The electron affinity of c-C4F8 was measured to be 24.2+/-2.3 kcal/mol (1.05+/-0.10 eV) by observing the equilibria for reaction of SF6-+c-C4F8=c-C4F8-+SF6. While the sound equilibrium for that reaction was established in the temperature region from similar to350 K down to the lowest temperature measured (similar to150 K), that was not established in the higher temperature region above 350 K. This was attributed to the existence of an isomer for c-C4F8-whose electron detachment energy is smaller than 1.05+/-0.10 eV. By the measurement of thermochemical stabilities of [(O-2)(m)(c-C4F8)(n)](-) (m+n=1-3), the lower electron affinity of c-C4F8 was determined to be 12.0+/-1.2 kcal/mol (0.52+/-0.05 eV). The lower limit of the proton affinity of c-C4F8 was estimated to be 130 kcal/mol.