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Thermal desorption analysis (TDA) is widely used to identify the trap site and the state of presence of hydrogen in steel.1) In a previous study we developed a finite difference computer program which simulates the spectrum of thermal desorption analysis (TDA) of hydrogen in steel.2) The simulation model was based upon the McNabb-Foster model3) with the hydrogen trap sites of discrete binding energies. The key parameters, e.g. trap energy and detrapping coefficient, were determined by fitting with the spectra of cylindrical specimens of 2.5 mm in radius by trial and error. From the value of the detrapping coefficient p0=25–30 s −1,2) it was thought that hydrogen desorption seemed to occur almost in the detrapping-controlled condition because the interval of detrapping events, i.e. inverse of p0, was calculated to be greater than the diffusion time to the surface of the specimen. The TDA spectrum is highly dependent on the specimen thickness. In a thin specimen lattice diffusion of hydrogen does not play a significant role in desorption because detrapped hydrogen almost instantly diffuses to the surface of the specimen. On the other hand, in a thick specimen lattice diffusion has a major influence on the spectrum shape because hydrogen atoms can escape from the trap site to such an extent that the hydrogen concentration in the lattice maintains equilibrium with that in the trap site. This is called diffusion-controlled desporption. If peak temperatures are measured at several ramp rates, the detrap energy of hydroInfluence of Specimen Thickness on Thermal Desorption Spectrum of Hydrogen in High Strength SCM435 Steel