We address the role of hydrogen in modifying the behaviour of screw dislocations in a-iron under the influence of the tensile and compressive stresses that are expected to arise in the vicinity of a crack tip. The quantum mechanical tight binding approximation is employed to calculate the variation of the hydrogen/screw dislocation interaction with strain. The locations and binding energies of hydrogen trap-sites surrounding the unstrained core are shown to be of similar accuracy to previous ab-initio results. Bi-axial tension and compression is applied normal to the dislocation line; the binding energies are found to vary linearly with strain. Estimations of the effect of hydrogen in strained regions are made based on the proposition that hydrogen may both enhance the nucleation of kink-pairs by lowering the formation energy, and slow the movement of kinks by a solute drag effect. Our results suggest that these effects will be enhanced in tensile regions. The chance of hardening by the collision of crossed kinks, to form a jog, is estimated by comparing the nucleation and annihilation rate of kinks on a straight screw dislocation for a range of hydrogen concentrations and temperatures, suggesting a greatly increased incidence with tension that is accentuated at lower temperatures. (C) 2020 The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.