Gas-phase reactions of NO+ with propylene (C3H6) were studied by using a pulsed electron beam mass spectrometer. An equilibrium between NO+ and NO+(C3H6)(1) was established. However, an NO+-initiated polymerization reaction forming [NO(C3H6)(n)](+) took place after n = 2, and the polymerized species NO+(C3H6)(n) up to n = 5 could be observed at the lowest temperature measured (102 K). The structures of [NO(C3H6)(n)](+) with n less than or equal to 5 were investigated by DFT calculations. There are cluster- and polymer-type isomers for n greater than or equal to 2. The latter has a long classical terminal C-C bonds (approximate to1.67 Angstrom) in [NO(C3H6)(n-1)](+) . . . C3H6 with a nitroso group at the other terminal.