Gas-phase reactions of NO + with propylene (C 3 H 6 ) were studied by using a pulsed electron beam mass spectrometer. An equilibrium between NO + and NO + (C 3 H 6 ) 1 was established. However, an NO + -initiated polymerization reaction forming [NO(C 3 H 6 ) n ] + took place after n=2, and the polymerized species NO + (C 3 H 6 ) n up to n=5 could be observed at the lowest temperature measured (102 K). The structures of [NO(C 3 H 6 ) n ] + with n=<5 were investigated by DFT calculations. There are cluster- and polymer-type isomers for n>=2. The latter has a long classical terminal C-C bonds (~1.67 Å) in [NO(C 3 H 6 ) n - 1 ] + ...C 3 H 6 with a nitroso group at the other terminal.