New Aurivillius phases in the Bi-Ag-Ti-O system were investigated by means of a solid-state reaction and X-ray diffraction. We found that the oxygen partial pressure has a significant influence on the synthesis of the Aurivillius phases. The mixed-layer Aurivillius phase Ag0.5Bi8.5Ti7O27 was observed after firing in an O-2 flow, but a single-phase material is difficult to obtain. A single-phase compound of the four-layer Aurivillus phase Ag0.5Bi4.5Ti4O15 was obtained on firing in an oxygen partial pressure of 10 bar (1 x 10(6) Pa). The dielectric properties (at 1 MHz) of the Ag0.5Bi4.5Ti4O15 compound were as follows: T-max = 687 degrees C, epsilon(r) = 166 (similar to 20 degrees C), and tan delta = 0.004 (similar to 20 degrees C).