This study explores the effects of particulate filler on the morphology of two immiscible high-molecular-weight thermoplastic polymers across a wide range of composition. Blends of polyisobutylene (PIB) and polyethylene oxide (PEO) with silica particle loadings of up to 30 vol% were studied. The silica particles have a strong affinity for PEO, and hence the effects of particles on the morphology depend on whether or not there is sufficient PEO to engulf the particles. We observe two morphologies that are qualitatively different from those seen in particle-free polymer blends: one in which particles are bonded together by small menisci of PEO, and the other in which a highly-filled particles-in-PEO phase percolates throughout the sample. Other morphologies in filled blends resemble droplet-matrix or cocontinuous morphologies in corresponding unfilled blends. Overall, particles have major effects on the morphology when the polymer preferred by the particles (PEO) is in a minority, but only modest effects when the preferred polymer is in a majority.