A three-year program of work supported by the European Commission (EC) involving eleven partners from six countries has been carried out to select, procure, characterize, and certify the thermal transport properties (thermal conductivity, λ, and thermal diffusivity, a) of Pyroceram 9606, a polycrystalline glass ceramic identified as the most suitable reference material for use up to at least 1000 °C. For the initial calibration, six blocks were chosen from a batch of 30 blocks purchased for the project. Measurements of the following properties were undertaken: chemical analysis and microstructure, density and porosity, homogeneity, anisotropy, thermal cycling between 20 °C and 1000 °C, and long-term stability and reproducibility. In addition, specific heat capacity, linear thermal expansivity, and thermal transmissivity measurements were carried out so that the thermal conductivity could also be determined from thermal-diffusivity values obtained from the certification measurements. As a result of a successful characterization program, the partners had no hesitation in recommending to the European Commission that full certification of the thermal conductivity and thermal diffusivity of Pyroceram 9606 should be undertaken.