The bonding of H to Fe is analyzed using qualitative band structure calculations in the framework of extended Hückel tight-binding theory and the ASED-MO cluster method. The changes in the electronic structure of bcc Fe upon the introduction of H at a vacancy are addressed, and a comparison drawn with H adsorption at the Fe(110) surface. H in bulk Fe with vacancies prefers a tetrahedral site shifted toward the vacancy. The vacancies act as a strong trap for H. The Fe atoms are initially more strongly bonded to each other as a consequence of vacancy formation; their Fe–Fe bond strength is then diminished as the new Fe–H bond is formed. The effect of H is limited to its first Fe neighbor. An analysis of the orbital interactions reveals that the Fe–H bonding involves mainly the Fe 4s and H 1s orbitals with less participation of Fe 4p and 3d crystal orbitals. Relaxation of Fe atoms neighboring the vacancy is also addressed. A detailed atomistic mechanism for decohesion and H embrittlement is provided.