Air plasma spray (APS) thermal barrier coatings (TBCs) contain pores, cracks and splat interfaces that are preferentially aligned normal to the heat flux direction. These significantly reduce (by as much as 50%) the thermal conductivity over a fully dense coating. Here the microstructures of APS thermal barrier coatings (TBCs) have been characterized in 3D using X-ray microcomputed tomography (-CT). Pores larger than 2.8m(3) are resolved and their contribution to thermal conductivity reduction is evaluated using image-based microstructurally realistic numerical models. However, the models overestimate (by almost 50%) the measured thermal conductivity of the as-deposited TBC sample. It is shown that this discrepancy is due to very fine (micrometer and submicrometer) cracks which readily sinter after short-term exposure to temperatures representative of TBC operating conditions causing the measured thermal conductivity to rise to within 15% of that predicted. This suggests that under the realistic service conditions, the 3D image-based models based on CT images provide a good indicator of the likely long-term TBC performance. Virtual experiments showed that for the retained pores, the larger pores which are flatter and more oriented in the plane of the APS splats, contribute disproportionately to the beneficial reduction in thermal conductivity. Our results demonstrate that X-ray imaging is a useful tool in establishing APS process conditions that give rise to a beneficial distribution of such pores.