A procedure is developed to estimate the effective thermal conductivity of chars of intumescent systems. The procedure is based on a heat transfer analysis of temperature-time data from one-dimensionally designed experiments of coated coupons exposed to a fire environment typical of aviation-type fuel fires. Thermal conductivities computed from both a first-order and second-order procedure are given, and are found to be within a factor of 2 or 3 of each other for the binders tested. Additionally, a simple theoretical approach, based on a one-dimensional equivalent thermal resistance model, yields estimates that are within a factor of three compared to the experimentally determined thermal conductivities. The conclusion of the theoretical model is that the low thermal conductivity of intumescent chars results from the insulating abilities of pockets of trapped gas within a porous char material, even though the char of an intumescent coating is a complex structure which has convective currents due to pyrolization of the binder.