As sustainability moves to the forefront of construction, the utilization of high-volume fly ash concrete mixtures to reduce CO2 emissions and cement consumption per unit volume of concrete placed is receiving renewed interest. Concrete mixtures in which the fly ash replaces 50% or more of the Portland cement are both economically and technically viable. This article focuses on a characterization of the thermal properties, namely, specific heat capacity and thermal conductivity, of such mixtures. Both the raw materials and the finished products (mortars and concretes) are evaluated using a transient plane source method. Because the specimens being examined are well hydrated, estimates of the specific heat capacity based on a law of mixtures, with a ‘bound water’ specific heat capacity value being employed for the water in the mixture, provide reasonable predictions of the measured performance. As with most materials, thermal conductivity is found to be a function of density, while also being dependent on whether the aggregate source is siliceous or limestone. The measured values should provide a useful database for evaluating the thermal performance of high-volume fly ash concrete structures.