Gas-phase reactions of NO + with propylene (C 3H 6) were studied by using a pulsed electron beam mass spectrometer. An equilibrium between NO + and NO +(C 3H 6) 1 was established. However, an NO +-initiated polymerization reaction forming [NO(C 3H 6) n ] + took place after n=2, and the polymerized species NO +(C 3H 6) n up to n=5 could be observed at the lowest temperature measured (102 K). The structures of [NO(C 3H 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 3H 6) n−1 ] +⋯C 3H 6 with a nitroso group at the other terminal.