The Aurivillius phases in the vicinity of the Bi5AgNb4O18 compound were determined by means of solid-state reaction, x-ray diffraction, and scanning electron microscopy. The stoichiometric Aurivillius phase Bi5AgNb4O18 is compatible with Ag/Ag2O, BiAgNb2O7, and BiNbO4. Two Aurivillius-phase solid solutions were found for the A-site-deficient Bi5+x/3Ag1−xNb4O18 and the oxygen-deficient Bi5−zAgzNb3O15−z. The x value of the Bi5+x/3Ag1−xNb4O18 solid-solution limit is between 0.2 and 0.4. A single-phase non-stoichiometric compound was obtained for x = 0.2 in Bi5+x/3Ag1−xNb4O18 and z = 0.25 in Bi5−zAgzNb3O15−z. The dielectric (at 1 MHz), ferroelectric, and piezoelectric properties of the stoichiometric Aurivillius phase Bi5AgNb4O18 were as follows: TC= 772 °C, ϵr= 197 (room temperature), ϵmax = 830, tanδ ∼ 10−3(room temperature to 540 °C), Pr = 6μC/cm2, Ec = 52kV/cm, d33 = 11pC/N.