The thermal diffusivity of plasma-sprayed zirconium oxide-7% yttrium oxide and aluminium oxide-3% titanium oxide ceramic coatings was measured by using a periodic heat-flow method. Samples were prepared with different porosities thus allowing the thermal diffusivity as function of porosity to be determined. The samples were also impregnated with silicone oil so that the effect on the thermal diffusivity by replacing air with silicone oil in the pores could be determined. The experimental results were compared with effective medium theories representing three different microstructures : (1) a continuous ceramic matrix with dispersed pores, (2) a continuous ceramic matrix with continuously interconnected pores, (3) dispersed ceramic particles loosely bonded together. The latter two microstructures gave the best agreement between the experimental data and the theory.