Silicon species are becoming emergent contaminants in the oil and gas industry due to their severe poisoning effect on the hydrotreatment (HDT) catalysts. Using an experimental pilot plant, fresh and representative samples of PDMS degradation under thermal cracking of hydrocarbons were produced. To follow the evolution of silicon species, the gas fraction was immediately analyzed by GC/TOFMS after the production and also after 4 months of storage at 4 degrees C. Cyclic siloxanes (D-n) as the major products of PDMS thermal degradation were characterized in the gas phase but these compounds are mainly present in the liquid fraction. Five volatile silicon compounds belonging to the families of silanes, siloxanes and silanols were characterized and quantified in the thermal cracking samples depending on the operating conditions applied in degradation tests. Under coking or visbreaking conditions (long residence time, absence of steam), silanes and siloxanes were preferentially formed. Under evaluated steam cracking conditions (short residence time and presence of steam), trimethylsilanol (TMSOH) was mainly produced by the hydrolysis of PDMS. The formation of the linear siloxane (L-2) after several month of storage at 4 degrees C by the self-condensation of TMSOH was also observed. The suspected poisoning effects of these molecules were discussed and could explain the deactivation of catalysts taking place in the refining of the light petroleum cuts. The new identified volatile silicon compounds could affect the performance of the catalyst by the reaction of hydroxyl groups potentially present at the surface of the support with reactive silicon molecules, more specifically silanols. (C) 2013 Elsevier Ltd. All rights reserved.