The phase stability of Aurivillius bismuth-layer structured Bi₅Ti₃FeO₁₅ (BTFO15) has been investigated in an epitaxial pillar-matrix nanocomposite system with spinel ferrites. Depending on the growth temperature a variety of super periodic structures of BTFOs appeared, denoted by the general formula BTFO15·nBiFeO₃ with almost continuous values of n between −0.5 and +1. In nanocomposites with CFO, n takes the positive values from n = 0, increasing up to n = 1 corresponding to Bi₆Ti₃Fe₂O₁₈ (BTFO18), as the growth temperature decreases. In contrast, in nanocomposites with NFO, n takes the negative values from n = 0, decreasing close to −0.5 corresponding to a well-known intergrowth structure of Bi₄Ti₃O₁₂ (BIT)–BTFO15, as the growth temperature increases. The formation mechanism of such super periodic structures is discussed in terms of the ordering of Bi₂O₂²⁺ layers in the perovskite blocks, driven by the Fe³⁺ transfer between BTFO15 and spinel ferrites as starting materials during phase separation, depending on the degree of excess or deficiency of the Bi amount.