Susceptibility of materials to Hydrogen Embrittlement depends critically on diffusivity and on those mechanisms involved in hydrogen transport within metals. Trapping phenomena are especially determinant for the accumulation of hydrogen in the Fracture Process Zone so they must be better understood and metallic defects must be characterised for a wide range of alloys and conditions. Thermal Desorption Analysis (TDA) is used here as a method for evaluating retention sites that are present in specimens made of a 42CrMo4 high strength steel with a tempered martensite microstructure. Experimental TDA curves show desorption peaks at different temperatures which are associated to different trapping sites. Varying the heating rate between different TDA tests, activation energies for the trapping – detrapping processes are found through a linear regression assuming the reaction proposed by Kissinger. However, limitations of this analytical procedure are critically exposed and its range of applicability is discussed. In order to overcome those limitations, a physically – based model considering kinetic equations proposed by McNabb and Foster is reviewed and implemented in a Finite Element software. Empirical and simulated curves are compared and, finally, a parametric study is carried out with the objective of estimating trap density and activation energy for each type of retention site.