The activation volume and dislocation velocity exponent have been obtained for polycrystalline iron in the extruded, extruded and annealed, and cold-rolled and annealed condition containing various amounts of alumina or zirconia particles, using the strain rate-change technique. It is found that the activation volume decreases and the dislocation velocity exponent increases, respectively, with decreasing interparticle spacing. The activation volume and dislocation velocity exponent have been examined on the basis of the various theories relating these parameters to the dislocation obstacles responsible for the lattice friction stress in b.c.c. iron. It is concluded that the basic dislocation-lattice interaction in iron is modified by the presence of particles; the presence of particles is considered to alter (directly or indirectly) the dislocation segment lengths and their critical bulge-size giving rise to the thermally activated flow stress contribution at room temperature. The dislocation velocity exponent also explains the yield-drop and Lüder's strain and is in a good agreement with Hahn's model.