Cyclic siloxanes (Dn) were confirmed as the major compounds of PDMS thermal degradation even in the presence of hydrocarbons with a relative amount generally around 95% or above. No significant difference on the formation of Dn were observed according to the different operating conditions under thermal cracking of hydrocarbons. For the first time, several other silicon compounds present at trace levels (<5% of the total area) were characterized. 伪,蠅-dihydroxy polydimethylsiloxanes, methylhydroxy cyclic siloxanes, (n + 1)oxasilabicyclo alcanes or bis(cyclosiloxanyl) siloxane, 伪-hydroxy, 蠅-methyl polysiloxanes and (n)oxasilabicyclo alcanes were preferentially formed in the presence of steam. Under evaluated coking or visbreaking conditions (long residence time without steam), other compounds were mainly characterized such as dimethoxy polysiloxanes, methyl(hydroperoxy) cyclic siloxanes, or ethoxy methyl cyclic siloxanes, linear polysiloxanes and methylpropyl cyclic siloxanes. Around one hundred silicon compounds were highlighted belonging to 12 different chemical families. The same repetition pattern (C2H6OSi), initially in PDMS, was present in all silicon compounds characterized in this study. Molecules with a number of silicon atoms ranging from 1 to 40 silicon atoms clearly demonstrated the occurrence of silicon in all petroleum cuts through very different chemical structures. This study shows that silicon species can distillate from gas fractions to the heavy petroleum cuts depending on their boiling points and chemical properties. These silicon species contain reactive groups (hydroxy, hydroperoxy, methoxy and ethoxy) which are able to react at the surface of HDT catalysts and cause a severe deactivation. Thus, these results appear as a crucial advance in progressing in the understanding of silicon poisoning.