The　apatite　crystal　structure　of　the　gadolinium　calcium　silicates　can　accommodate　a　wide　range　of　point　defects,　including　oxygen　and　cation　vacancies,　as　well　as　anti-site　defects,　depending　on　the　Gd/Ca　ratio.　Compositions　having　only　cation　or　oxygen　vacancies　were　identified　and　the　thermal　diffusivity　and　conductivity　were　measured　up　to　1000　°C.　All　the　compositions,　including　the　stoichiometric　composition,　exhibit　low　thermal　conductivities　from　room　temperature　to　high　temperature　with　the　defect-containing　compositions　having　even　lower　thermal　conductivities.　The　high-temperature　thermal　conductivity,　at　temperatures　below　the　onset　of　significant　radiative　heat　transport,　decreases　with　the　inverse　square　root　of　the　cation　and　anion　vacancy　concentration,　consistent　with　simple　defect　scattering　models.　Based　on　the　data,　it　is　concluded　that　the　oxygen　vacancies　are　slightly　more　effective　in　reducing　thermal　conductivity.　©　2011　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.