Polymer-coated nanoparticle additives are shown to stabilize the formation of high- energy tilt and twist grain boundary structures in amorphous lamellar block copolymer/ nanoparticle blends. The distribution of the particle additives within the grain boundary region depends on the level of perturbation of the equilibrium structure and presents analogies to previously described block copolymer/ homopolymer blends. At small tilt angles ( chevron grain boundary) the particle distribution is equal to the equilibrium distribution ( here: center location of PS- coated gold particles within the PS domains of a PS-PEP block copolymer) whereas at larger tilt angles ( omega or T-junction grain boundary structure) the particles are found to selectively swell the high elastic energy regions along the grain boundary, thereby stabilizing the formation of otherwise energetically unfavorable grain boundary structures presumably through the relaxation of chains in the grain boundary region. The direct visualization of the swelling process provides the first experimental evidence for the mechanism of stabilization of energetically unfavorable grain boundary structures by selective swelling and stress relief that was previously postulated for block copolymer/ homopolymer blends and presents a model system for comparison with theoretical predictions.