The relief of lattice mismatch-induced strain in Bi(111) on Si(001) heteroepitaxial system was investigated in real time as the Bi film relaxes, by means of high resolution low-energy electron diffraction (LEED). The inherent lattice mismatch of 2.5% at room temperature is accommodated through the formation of an ordered misfit dislocation array confined to the interface. The strain fields of the dislocations cause a periodic height undulation of the surface in the sub-Angstrom regime, which is observed through spot splitting in LEED. From a simulations measurement of the position of the first-order LEED spots, which corresponds to the lattice parameter of the film, and of the separation of satellite spots, which corresponds to the ordering of the dislocation array, the evolution of the strain state during annealing of a 6 nm Bi film was determined. The strain is solely relaxed by full edge-type dislocations arranged in the ordered array at the interface. From the remaining strain of epsilon = 0.6% the critical thickness for generation of misfit dislocations under equilibrium conditions can be derived. (C) 2014 Published by Elsevier B.V.