Crystal types from 3 oak species representing 3 different sections of the genus Quercus were identified. Crystals were studied with scanning electron microscopy after localisation by light microscopy. The crystals were composed of calcium oxalate silicates as whewellite (calcium oxalate monohydrate) composites. In Quercus macranthera Fisch. et Mey. subsp. syspirensis (C.Koch) Menitsky (Ispir oak) from the white oaks (section Quercus: Leucobalanus), whewellite and trihydrated weddellites coexisted. Axial parenchyma cells included 30 crystal chains with walls as chambers in uniseriate ray cells; some cells had many crystals without any chambers and were small. Long thin crystals were found adherent to the membrane in the tracheary cell lumens of latewood. Quercus cerris L. var. cerris (hairy oak/Turkey oak) from the red oak group (section Cerris Loudon) is extremely rich in rhomboidal crystals. In species Quercus aucheri Jaub. et Spach (grey pirnal) from the evergreen oak group (section Ilex Loudon) rhomboidal crystals Were found in axial parenchyma and multiseriate ray cells. Lignified wall layers were not observed around the crystals with chambers.