New substituted Aurivillius phases of general compositions Bi2-xTlxSrNb2O9-x, Bi2-yTlySr1-2yLa2yNb2O9, and Bi2-zTlzSr1-zLazNb2 O9-z/2 have been synthesized. The extent of the substitution in each solid solution has been correlated with the size of the cations and the simultaneous creation of oxygen vacancies in the network. Steric considerations suggest the preferential location of oxygen vacancies in the bismuth, thallium-oxygen layer. Impedance spectroscopy measurements have shown a similar electrical behavior of oxygen-deficient and nondeficient phases; this fact has been explained as due to the high energy barriers to oxygen ion migration in the bismuth layer of Aurivillius phases.