We investigated the effects of substitutional alloying elements on the microstructure, hydrogen diffusivity, and tensile properties of Fe–X binary ferritic alloys (X = Si, Al, Mn, Cu, Ni, Co, Cr, Mo, V, W, and Ti) in air and under hydrogen charging. We find using X-ray diffraction that these elements, except for Si and Co, cause ferrite lattice expansion. The hydrogen diffusion coefficient D (measured via hydrogen-permeation tests under cathodic charging at 24°C) reduces as a function of the added alloy concentration. The D-value reduction is enhanced more for Ti, Mn and Cr than other elements. This D variation cannot be simply explained based on the lattice expansion effect, which means that D depends on both hydrogen trapping at the expanded internal lattice spaces adjacent to substitutional solute atoms and hydrogen-solute-atoms chemical interactions. As regards the tensile properties obtained based on slow strain rate tests in air and under hydrogen charging, we find that the all elements, except for Al and Co, afford alloy strengthening in air. Under hydrogen charging, Ti, Mn, and Cr addition reduces the fracture elongation, thereby indicating that these elements increase alloy susceptibility to hydrogen embrittlement. The elongation loss due to hydrogen does not depend on the strengthening effects; however, it exhibits good correlation with the observed D-value reduction and the increment in surface hydrogen concentration C0, which is inversely proportional to D. This correlation indicates that substitutional alloying elements act as reversible hydrogen-trapping sites, which supply hydrogen to potential and developing cracks.