The size and modulus misfit between solute and parent atoms gives rise to strengthening, solid solution hardening (SSH). With the development of Argon’s expression for the interaction energies for solute atoms and dislocations for size and modulus misfit, both effects can now be modeled without the introduction of adjustable or arbitrary parameters. These expressions are used to derive models for SSH during creep. Although the constants for the modulus misfit can be an order larger than those for size misfit, the latter effect is still dominating. The interaction energy gives a direct contribution to the activation energy for creep. The solutes form Cottrell atmospheres around the dislocations. For slowly diffusion elements, these atmospheres give rise to a drag force that slows down the motion of the dislocations. Fast diffusing elements have to break away from the dislocations to enable their motion. This creates a break stress that is the source of SSH in this case.