The utilization of a slug calorimeter to evaluate the thermal performance of fire resistive materials (FRMs) is presented. The basic specimen configuration consists of a "sandwich", with a square central stainless steel plate (slug) surrounded on two sides by the FRM. This sandwich configuration provides an adiabatic boundary condition at the central axis of the slug plate that greatly simplifies the analysis. The other four (thin) sides of the steel plate (and FRM specimens) are insulated using a low thermal conductivity fumed-silica board. Two metal plates manufactured from a high temperature alloy provide a frame for placing the entire sandwich specimen slightly in compression. The entire configuration is centrally placed at the bottom of an electrically-heated box furnace and the temperatures of the metal slug and exterior FRM surfaces are monitored during multiple heating and cooling cycles. Knowing the heat capacities and densities of the steel slug and the FRM, an effective thermal conductivity for the FRM can be estimated. The effective thermal conductivity of the FRM will be influenced by its true thermal conductivity and by any endothermic or exothermic reactions or phase changes occurring within the FRM. Preliminary tests have been conducted on two commonly used FRMs and on a non-reactive fumed-silica board to demonstrate the feasibility of the method.