Lignin is the only renewable bio-resource of aromatic compounds [2]. Due to the large depletion on fossil resources, develop successful processes which enable to convert lignin into high value chemicals (aromatic chemicals platform) are highly leading [1], [2]. Owing to its complex structure, the challenge is high: it consists in selectively breaking down lignin main bonds between building blocks [1-3] and defunctionnalized it to the target products. The hydroconversion process of wheat straw lignin in presence of tetralin, high pressure of hydrogen (8 MPa) and heterogeneous catalyst was preliminary studied [2]. During the reaction, the lignin was thermally depolymerised and the role of hydrogen, catalyst and solvent partially consisted in saturating the produced radicals. The results showed that the solvent nature had a great impact on reducing condensation reactions and residues formation. However, the lignin behaviour, i.e. its physical or reactive solubilisation in the solvent medium, was not yet elucidated. Moreover, the impact of solvent nature on the first steps of lignin depolymerisation was not fully understood. To clarify these points an experimental setup was built up with the support of ANR Lignarocat project [5]. It consisted in a batch reactor (0.5L) which operated at elevated pressure (<130 bar) and high temperature (<400 °C). It was equipped with in situ on-line ATR (Attenuated Total Reflectance) –IR probe which led to detect and follow mass transfer of lignin into solution in the process operating conditions. To simplify the experimental study, lignin depolymerisation was firstly performed in solvolyse conditions, i.e. at lower temperature and without catalyst and gaseous hydrogen addition. The objective was to understand which mechanism dominated (direct solubilisation or reactive depolymerisation) at lower temperature (below 250°C). The system characterization was completed by analytical tools off-line (GPC). Hydroconversion process conditions will be studied lately.