The double-layer Aurivillius phases Pb1-2xBi1.5+2xLa0.5Nb2-xMnxO9 (x = 0, 0.1, 0.3, and 0.5) were synthesized by a molten salt method using a mixture of K2SO4 /Na2SO4. The effect of composition on the structure, morphology and dielectric properties was investigated. X-ray diffraction showed that single-phase samples with a non-polar, orthorhombic A2(1)am structure were obtained for x = 0, 0.1 and 0.3. The unit cell becomes more orthorhombic with increasing x as the degree of distortion of the BO6 octahedra in the perovskite layer increases. Raman spectroscopy showed the typical modes of the orthorhombic double-layer Aurivillius structure and indicated that the La3+ ions prefer to occupy the perovskite A-site, Mn3+ occupies the B-site, and the Pb2+ ions are found in the Bi2O2 layer. The morphology of the samples was probed by scanning electron microscopy, which showed anisotropic, plate-like crystallites that increased in size with increasing x. The dielectric constant significantly increased with x, and the ferroelectric properties became more relaxor-like.