Gas-phase ion-molecule reactions in hexafluoropropene (C3F6) were studied with a pulsed electron beam mass spectrometer. In the reactions Of C3F6+ and C2F4+ ions with C3F6 to form C3F6+(C3F6)(n) and C2F4+(C3F6)(n), respectively, the equilibria could not be observed but the cluster ions C3F6+(C3F6)(n) and C2F4+(C3F6)(n) were found to grow very slowly at the expense of the smaller cluster ions C3F6+(C3F6)(n-1) and C2F4+(C3F6)(n-1), respectively. The decay rates of C3F6+(C3F6)(n-1) and C2F4+(C3F6)(n-1) to form C3F6+(C3F6)(n) and C2F4+(C3F6)(n), respectively, become faster at lower temperature. This is due to the existence of an entropy barrier for the formation of cluster ions. The C3F5+ ion was found to form cluster ions readily with C3F6 solvent molecules. Thermochemical stabilities for C3F5+(C3F6)(n) with n = 1 and 2 could be determined. The proton affinity (PA) of C3F6 was found to be smaller but close to that of C2H4 (162.6 +/- 1.5 kcal/mol). The G2MP2-calculated PA is 157.26 kcal/mol. Lone-pair orbitals of the CF3 substituent are electronic-charge donor sites to C2F4+ and C3F5+. The polymerization reactions of C3F6 initiated by F-, C3F5-, and C3F6- were observed. Those reactions became faster with a decrease of temperature. The high reactivity of C3F6 in the negative-mode ion/molecule reactions is ascribed to the perfluoro effect. The halide ions Cl-, Br-, and I- were found to form cluster ions with C3F6. Thermochemical stabilities for X-(C3F6)(n) (X- = Cl-, Br-, and I-) have been determined. A slight charge transfer in the complex Cl- --> C3F6 results in the fairly strong bond energy (12.6 kcal/mol) for the cluster.