The mechanical properties and structure of composite films made of high-T-g polystyrene (PS) nodules dispersed in a low-T-g poly(butyl acrylate) (PBuA) matrix were studied by means of dynamic mechanical spectrometry and small-angle neutron scattering. At temperatures between the T-g of PS and PBuA, the mechanical properties depend drastically on the spatial distribution of the glassy PS domains. For films cast from mixtures of PS and PBuA latexes, an aggregation of PS particles into dense clusters dispersed in the PBuA matrix was observed. As a consequence, a large density of contacts between PS particles in the dry films ensured a strong mechanical reinforcement above a percolation threshold of 30% PS volume fraction. Extensive coalescence of PS particles occurred at their contacts upon annealing the films above the T-g of PS, leading to further mechanical reinforcement. For films cast from core-shell particles, the aggregation phenomenon was prevented, depending on the coverage of the PS core by the PBuA shell. An efficient core encapsulation in the core-shell morphology led to poor contact between PS cores, the elastic moduli were then close to that of the PBuA matrix, and the PS coalescence upon annealing was prevented.