Platelet-shaped particles of similar size and shape were investigated as fillers for improving the thermal conductivity of polymer- ceramic composite materials. The conductivities of composites filled with hard, stiff ceramic particles exceeded 3.5 W.(m.K)(-1), or >20 times the conductivity of the polymer matrix, and were shown to be almost independent of the intrinsic filler conductivity range of 33-300 W.(m.K)(-1). In contrast, the thermal conductivity of composites filled with soft, platelet-shaped BN fillers reached over 13 W.(m.K)(-1). A mechanism is proposed whereby deformation of the soft filler particles provides improved particle-to-particle connectivity and allows greater packing density, resulting in the ability to achieve much higher conductivity than is possible for hard and stiff particles of similar initial morphology. Experimental results are discussed in light of various thermal conductivity prediction models in the literature.