The mineralogical characteristics of some Turkish opal-CTs collected from West and Middle Anatolia were studied. They are originated from Eocene to Upper Miocene aged volcanic-sedimentary formations. Of the sixteen samples studied, all except one, were found to be optically isotropic. 101-spacings (4 Angstrom band) of opal-CTs show differences within the range of 4.086-4.144 Angstrom. There are proportional relationships between 4 Angstrom band, 2.5 Angstrom line, density and total weight loss values of opal-CTs. As their structural disorder which is observed in 4 Angstrom band values increases, the water content and the density of opal-CTs increase. The structural ordering increases depending on the geological ageing. Opal-CTs of the samples which consist of opal-CT and quartz associations have more ordered structure than those of opal-CTs which are the only component of the samples.