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Polybutadiene (PB) and Polyisoprene (PI) the two most common polydienes (PD), are involved in a large number of materials and used in a wide variety of applications. The characterization of these polymers by mass spectrometry (MS) continues to be very challenging due to their high insolubility and the difficulty to ionize them. In this work, cross-metathesis reaction was used to generate end-functionalized acetoxy ionizable oligomers for the structural deciphering of different commercial PB and PI samples. A cross-metathesis reaction was carried out between polymers and the Z-1,4-diacetoxy-2-butene as chain transfer agent in dichloromethane using Hoveyda-Grubbs second-generation catalyst. Well-defined acetoxy telechelic structures were obtained and analysed by Fourier-Transform ion cyclotron resonance (FTICR) high resolution MS. However, after depolymerization, low molar mass polyolefins contained some units with different configurations, suggesting an olefin isomerization reaction due to the decomposition of the catalyst. The addition of an electron-deficient reagent such as 2,6-dichloro-1,4-benzoquinone suppressed this isomerization in the case of both Z- and E- PB and PI. Ion-mobility spectrometry-mass spectrometry (IMS-MS) and energy resolved tandem mass spectrometry (ERMS) analyses confirmed a successful isomerization suppression. For comparing the results obtained by depolymerisation with classical methods for polymer analysis, pyrolysis-comprehensive two-dimensional gas chromatography/mass spectrometry (Py-GC×GC-MS), atmospheric solid analysis probe (ASAP) and DIP-APCI analyses were performed on …