In the oil and gas industry, silicon coming from antifoaming agents, is recognized to be an emergent contaminant for hydrotreatment catalyst due to its negative effect on their performance. Using a pilot plant, representative samples of the thermal degradation of antifoaming agent such as polydimethylsiloxanes (PDMS) were produced under evaluated thermal cracking of hydrocarbons with the presence or absence of steam. Micro size exclusion chromatography hyphenated to inductively coupled plasma high resolution mass spectrometry was used to characterize these samples. Under low temperature processing conditions at around 250 degrees C, PDMS was not degraded in the presence of hydrocarbons. Under thermal cracking of hydrocarbons at 500 degrees C, PDMS is degraded and formed intermediate polymers and low molecular weight compounds. A small amount of the initial PDMS can be present following the different operating conditions. The percentage of steam has a great influence on the degradation of intermediate polymers probably following a hydrolysis to produce low molecular weight species. The results presented bring preliminary results and a new insight on the degradation of PDMS under thermal cracking conditions. This work indicates that the different molecular weight silicon compounds can distillate in the various petroleum cuts and could be responsible for catalyst poisoning. (C) 2012 Elsevier B.V. All rights reserved.