Within a broader study of epoxy-thiol “click” systems [1], we are interested here in the fabrication of epoxy-thiol composites filled with boron nitride (BN) for use as insulated metal substrates. For such an application, the material must display high thermal conductivity and low electrical conductivity. In this work we examine the effect of the BN filler on the epoxy-thiol curing reaction and on the thermal conductivity of the fully cured samples. Composite samples are prepared by mechanically mixing the BN particles, in the desired proportion (up to 34 vol%), with the stoichiometric amounts of epoxy (DGEBA) and thiol (pentaerythritol tetrakis) and an initiator (encapsulated imidazole). The BN particles are obtained commercially (Saint Gobain) in different forms: hexagonal platelets of sizes 2 μm and 6 μm, and spherical agglomerates of size 80 μm. The cure reaction is monitored by differential scanning calorimetry (DSC) in both isothermal and non-isothermal mode, and the thermal conductivity of cured samples is measured by the Transient Hot Bridge method [2] using the HotPoint sensor calibrated with 5 different materials over the range from 0.2 to 20 W/mK. The DSC results show that the cure reaction is first accelerated by a low vol% of BN and is then retarded at higher BN contents; this is evident from the peak exotherm temperature in nonisothermal cure and in the time to the peak exotherm in isothermal cure. The effect is more pronounced with the 80 μm BN agglomerates, and even more so with a hybrid of 80 μm agglomerates with 2 μm or 6 μm platelets. Despite the change in the cure kinetics with the addition of BN filler, the heat of reaction and the glass transition temperature of the fully cured systems is always the same. Furthermore, the effect is not observed when the epoxy is cured with an amine. Accordingly, we attributed it to a physical effect associated with the heat transfer into the sample, which appears to be better in the epoxy-thiol system, and would therefore anticipate an enhanced thermal conductivity in the composites with 80 μm agglomerates and in the hybrids. The measurements of thermal conductivity show that this is indeed the situation. For all samples, the thermal conductivity increases with BN content in a non-linear manner, but the increase is greatest for the hybrids, followed by the 80 μm agglomerate samples. For example, for the hybrid with 34 vol% BN, the thermal conductivity is 4.0 W/mK. The thermal conductivities of the samples with 6 μm and 2 μm platelets are somewhat lower, but nevertheless greater than those obtained with an epoxy-diamine matrix. The enhanced thermal conductivity is attributed to an improved matrix-filler interaction in the epoxy-thiol composites, which could be associated with Lewis acid-base coordination.