Abstract : The gasification of biomass by thermochemical processes is presently a promising method of producting H2 and CO which are used for synthesis of biogasoil. However, the thermal treatment of biomass leads to the production of unwanted compounds referred to as "tar" (heavy hydrocarbons) of which the content in syngas must to be inferior to 0.1 mg.Nm-3. Knowledge of tar content is consequently needed. To this end, a sampling methodology using SPME (Solid Phase MicroExtraction) was developed for the quantification of "model compounds" which represents the total tar content mainly emitted by two gasification or pyrolysis processes. Introduction of the notion of model compounds is intended to simplify the sampling methodology on one hand to overcome the possible limitations related to the sampling system developed in this work to allow SPME extraction on the other. For the two thermochemical processes, 15 model compounds were selected. The quantification of these compounds required calibration of SPME fibres (polydimethylsiloxane coating) which allow us, over a range of temperatures, to establish the absorption profiles and to determine distribution coefficients. Influence of the sampling conditions on the absorption and performance of the coupled SPME-gas Chromatography were also evaluated. In order to avoid repetitive calibrations, a model based on thermodynamic considerations was developed to speed-up determination of the distribution coefficient.