α-Iron single crystals have been investigated in the (101)  slip system between 123 and 296 K up to 20 % deformation. Stress relaxation and creep tests have been used to measure activation parameters during tensile straining. Berg-Barrett X-ray reflection topography was used to examine the overall dislocation substructure. Below 200 K the observed dislocation substructure is homogeneous ; the activation volume is independent of strain and the stress-strain curves show an almost zero work-hardening rate : the whole plasticity is controlled solely by the Peierls friction. Above 200 K the dislocation substructure becomes more and more heterogeneous as temperature increases ; the activation volume decreases with strain and the work-hardening rate is around 10 -3 μ : thermal activation overcomes the lattice friction.