During the summer of 1996, wideband (0.003125--2000 s) magnetotelluric (MT) data were collected from 52 sites across western Turkey, with a site spacing of ~5 km. The extensional regime is dominant as a result of the convergence of the African and Arabian plates against the Eurasian plate, and western Turkey is characterized with east-west oriented, continental escape tectonics. To contribute to the knowledge of the geological structures along the profile, the dimensionality characteristics of the MT impedance tensors are computed, taking real and imaginary parts of the tensor elements separately. The rotationally invariant parameters of central impedance (d3) and anisotropy angle (λ) are also computed, the two being good dimensionality indicators. Pseudosections of these parameters reveal the presence of major geological structures in western Turkey, such as the zone between the Menderes Massif and the Bornova Flysch zone, the Izmir-Ankara Suture zone, the western part of the North Anatolian fault zone, and grabens (Demirci, G¿rdes, and Bigadi¿), which are the characteristic of the region. Interpretation of the full set of Mohr circles shows a strong anisotropy and high central impedance anomalies, while changes are shown in geological strike direction at estimated depths of about 7--8 km, 15--20 km, and 35--40 km along the profile from south to north. These anomalies are indicative of changes in the thickness of upper crust. ¿ 2000 American Geophysical Union