  We consider the effects of interactions on the nature of spinon excitations
in Heisenberg spin-1/2 chains. We explicitly compute the two-spinon part of the
longitudinal structure factor of the infinite chain in zero field for all
values of anisotropy in the gapless antiferromagnetic regime, via an exact
algebraic approach. Our results allow us to quantitatively describe the
behaviour of these fundamental excitations for cases ranging from free to fully
coupled chains, thereby explicitly mapping the effects of `turning on the
interactions' in a strongly-correlated system.
