In this study, the influence of a metallic catalyst (Pt/C) on the direct depolymerization of the lignin contained in Kraft liquor (without its precipitation) has been studied. After the depolymerization reaction at the optimal conditions, three main products were obtained: oil (with phenolic monomers), residual lignin and char. The first one has been characterized by GC/MS to quantify the phenolic monomers present in the oil. An experimental response surface design was made using the Statgraphic Centurion version XV software to analyse the influence of the catalyst concentration and the reaction time in both oil yield and the phenolic monomers production from the lignin. The catalyst concentration and reaction time have been varied between 0 - 4 wt.% and 20 - 80 min. All the depolymerization processes have been carried out at 300 ºC of temperature and 90 bar of pressure. After the experimental design, an optimum point has been obtained where the yield of oil and monomeric products were maximum. The validation of the model has been carried out by performing the experiments at the optimal conditions and comparing them with the values predicted by the design.