Gas-phase ion-molecule reactions in octafluorocyclopentene (C5F8) were studied with a pulsed electron beam mass spectrometer. When a few Torr of major gas, CH4, Ar, or N2, containing approximately 10 mTorr C5F8 was ionized by 2 keV electrons, C5F8+, C5F7+, C4F6+, C4F5+, and C3F3+ were formed as major fragment ions. The interaction between those ions and C5F8 is found to be a weak electrostatic interaction. The cation...C5F8 bonding energies are around 10 kcal/mol, which were reproduced well by (U)B3LYP/6-311+G(d) calculations. The proton affinity of C5F8 (=148.6 kcal/mol by B3LYP/6-311+G(d)) was found to be smaller than that of C2H4 (=162.8 kcal/mol). In the negative mode of operation, the intense signal of C5F8- was observed during the electron pulse. This indicates that C5F8 has a positive electron affinity (1.27 eV by (U)B3LYP/6-311+G(d)). The C5F8- ion was quickly converted to a complex C10F16-. This complex did not react further with C5F8 down to 170 K. The theoretical calculation revealed that a C5F7-F-...C5F8 interaction mode in (C5F8)2- was converted to a C5F7*...C5F9- one via fluoride-ion transfer. The F- ion was found to form a strong covalent bond with C5F8, but the interaction in F-(C5F8)- - -C5F8 is a weak electrostatic interaction due to the charge dispersal in F-(C5F8). The halide ions except F- interact with C5F8 only weakly. Thermochemical stabilities for the cluster ions I-(CH3I)n (n = 1, 2) were also determined.