In this paper, the effect of local hydrogen concentration and distribution in magnesium (Mg) alloys is studied in regard to hydrogen embrittlement. Quantitative studies of hydrogen trapping sites and release behavior in AZ91 and AZ31 magnesium alloys are being studied by thermal desorption analysis (TDS). The trapping energy levels are used to discuss the embrittlement mechanisms due to their control on hydrogen availability. The embrittlement process is caused by hydrogen in combination with residual or applied stress and can lead to the mechanical degradation of a material. The susceptibility of Mg alloys is directly related to the role of the second phases controlling the hydrogen trapping mechanisms. In this work, we examine the effect of Mg's microstructure on the magnesium hydride (MgH2) reaction, referred to as hydriding, and its decomposition, referred to as dehydriding. The MgH2 compound was investigated in regard to two aspects: first, as the main source for controlling the hydrogen dehydriding process; second, as a hydrogen trapping site for preventing hydrogen embrittlement process. The TDS analysis was used to study the hydrogen trapping mechanisms by studying the traps' density and distribution and relating them to potential lattice defects. The TDS analysis revealed a certain hydrogen concentration evolving near beta-Mg17Al12 phase, accompanied by H-2 desorption at a temperature range between similar to 200 and 300 degrees C. It is proposed that beta-phase plays a fundamental role in the dehydriding process, and this response is a crucial step in effecting the embrittlement behavior.